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University  of  California  •  Berkeley 


FttOM         *         ^    T> 
IV.    A.   li  GARY'S      (> 

CIIEVP   BOOK  STOIIE,         ()i 

<o.  158  ISorth  Second  Strect'i 
corner  of  New.      Philad' 


Philad'a-  61 


ELEMENTS 


OF 


NATURAL  PHILOSOPHY; 


EXPLAINING 


THE  LAWS  AND  PRINCIPLES 


OF 


ATTRACTION, 
GRAVITxVTION, 
MECHANICS, 
PNEUMATICS, 


HYDROSTATICS, 
HYDRAULICS, 
ELECTRICITY, 
AND  OPTICS: 


WITH   A  GENERAL  VIEW  OF 

THE  SOLAR  SYSTEM. 

ADAPTED  TO  PUBLIC  AND  PRIVATE  INSTRUCTION 

BY  JOHN  WEBSTER. 

WITH  NOTES  AND  CORRECTIONS, 
BY  ROBERT  PATTERSON, 

I'ROrBSSOROF  MATHEMATICS  IN  THE  UNIVERSITY  OF  PENNSYLVANIA, 

PHILADELPHIA: 

PUBLISHED  BY  B.  AND  T.  KITE,  NO.  20,  NORTH  THIRD-STREET. 
FRY  AND  KAMMERER,  PRINTERS. 

1808 


District  of  Pennsylvania,  to  wit  : 

BE  IT  REMEMBERED,  That  on  the  nineteenth  day  of  February, 
in  the  thirty-second  year  of  the  Independence  of  the  United 

SEAL.    States  of  America,  a   d.  1808,  Benjamin  and  Tliomas  Kite, 
of  the  said  District,  have  deposited  in  this  Office,  the  title  of 
a  book,  the  right  whereof  they  claim  as  Proprietors,  in  the  words  fol- 
lowing-, to  wit: 

**  Elements  of  Natural  Philosophy;  explaining  the  laws  and  princi- 
ples of  Attraction,  Gravitation,  Mechanics,  Pneumatics,  Hydrostatics, 
Hydraulics,  Electricity,  and  Optics:  with  a  general  view  of  the  Solar 
System  Adapted  to  public  and  private  instruction.  By  John  Webster. 
With  Notes  and  Corrections,  by  Robert  Patterson,  Professor  of  Mathe- 
matics in  the  University  of  Pennsylvania.'* 

In  conformity  to  the  act  of  the  Congress  of  the  United  States,  inti- 
tuled, '*  An  act  for  the  encouragement  of  learning,  by  securing  the 
copies  of  maps,  charts,  and  books,  to  the  authors  and  proprietors  of 
such  copies  during  the  times  therein  mentioned  :"  And  also  to  the  act, 
entitled,  **  An  act  supplementary  to  an  act,  entitled,  *  An  act  for  the 
encouragement  of  learning,  by  securing  the  copies  of  maps,  charts,  and 
books,  to  the  authors  and  proprietors  of  such  copies  during  the  times 
therein  mentioned,'  and  extending  the  benefits  thereof  to  the  arts  of 
designing,  engraving,  and  etching  historical  and  other  prints." 

D.CALDWELL, 
Clerk  of  the  District  of  Pennsylvania. 


TO 

MR.  JOHN  BONNYCASTLE, 

MATHEMATICAL  MASTER  AT  THE  ROYAL  ACADEMY,  WOOLWICH; 

THIS  WORK 
IS  RESPECTFULLY  INSCRIBED, 

FROM 

A  REGARD  TO  HIS  TALENTS, 

AND  AN  ESTEEM  FOR  HIS  VIRTUES, 

BY  THE  AUTHOR. 


ic;  ,11  lie 

■thfund 


http://www.archive.org/details/elementsofnaturaOOwebsrich 


PREFACE. 

The  great  object  of  science  is  to  ameliorate  the 
condition  of  man,  by  adding  to  those  advantages  which 
he  naturally  possesses.  What  would  avail  the  deep  and 
speculative  inquiries,  by  which  the  learned  attempt  to 
trace  the  source  of  Infinite  Wisdom,  if  their  labours 
did  not  produce  benefit  to  their  fellow  creatures? 

In  this  vie^v  the  study  of  Philosophy  stands  highly 
conspicuous,  not  only  in  the  pleasure  it  aiFords  in  the 
pursuit,  but  in  promoting  our  interests,  supplying  our 
necessities,  and  adding  to  the  general  happiness  of 
mankind. 

It  is  not  a  part  of  society,  but  the  whole,  that  is  in- 
terested in  this  kind  of  information;  for  well-grounded 
philosophy  is  the  parent  of  arts,  commerce,  and  agri- 
culture, which  are  the  vital  principles  that  promote  the 
wellbeing  of  civilized  states.  Nor  is  it  less  efficacious 
in  fixing  the  principles  of  religion:  the  more  compre- 
hensive our  view  of  the  divine  productions  in  the  cre- 
ation of  the  universe,  the  more  strong  and  lasting  will 
be  our  conviction  of  the  power,  wisdom,  and  goodness, 
of  the  great  Author  of  all  things. 

If,  then,  philosophical  knowledge  be  of  such  es- 
sential advantage  in  the  general  pursuits  of  society,  it 


vi  PREFACE. 

surely  becomes  highly  expedient  to  diffuse  it  in  such 
a  manner,  as  to  enable  every  class  to  obtain  some  por- 
tion of  the  whole. 

As  a  number  of  learned  works  have  been  written 
on  this  subject,  which  are  only  calculated  for  persons 
of  leisure  and  education;  it  cannot  be  an  unworthy 
attempt  to  gather  the  fruits  of  these  labours,  and  adapt 
them  to  more  general  use.  It  is,  therefore,  the  humble 
endeavour  of  the  author  of  this  work,  to  collect  and 
methodize  those  demonstrative  truths,  which  have 
been  drawn  from  the  bosom  of  nature  by  the  deep  re- 
searches of  the  philosopher,  and  to  render  them  plain 
and  evident  to  those,  whose  time  and  education  will 
not  enable  them  to  draw  their  information  from  origi- 
nal sources. 

By  the  exertions  of  able  experimentalists,  the  love 
of  physical  science  has  been  greatly  extended ;  and  the 
general  class  of  society  has  become  more  interested  in 
its  pursuit,  from  a  well-directed  view  of  its  utility. 
But  even  public  lectures  lo^e  their  effect  upon  a  con- 
siderable part  of  the  auditors,  from  the  want  of  prepa- 
ratory knowledge.  If  outlines  of  the  subjects  were 
previously  fixed  in  the  mind,  by  an  easy  introductory 
work,  the  hearer  would  be  prepared  for  the  subject, 
and  would  gain  much  greater  advantage  from  the 
lecture. 

More  than  one  half  of  the  young  people  who  ai'e 
placed  in  public  schools,  are  intended  for  those  com- 
mon avocations  in  life,  which  leave  but  a  circumscribed 


PREFACE.  vii 

portion  of  time  to  attain  the  various  objects  of  educa- 
tion. It  is,  therefore,  neither  to  be  expected,  nor  is  it 
intended,  that  they  should  acquire  any  thing  more  than 
a  general  knowledge  of  science.  The  first  considera-- 
tion  then  is,  how  to  employ  this  small  portion  of  time 
in  such  a  manner  as  to  produce  the  greatest  advantage 
to  the  pupiL  If  it  be  admitted,  that  it  is  an  object 
worthy  of  attention  to  instruct  the  youthful  mind  in 
physical  knowledge,  and  to  extend  philosophy  to  the 
useful  purposes  of  life,  the  subject  will  require  such 
an  arrangement,  that  its  acquisition  may  be  rendered 
compatible  with  the  time  and  ability  of  the  pupil. 

It  is  hoped,  that  the  following  pages  will  not  be 
found  totally  inadequate  to  this  desirable  purpose. 
Speculative  theory  and  mathematical  demonstrations 
have  been  as  much  avoided  as  possible,  to  make  way 
for  those  useful  and  evident  truths,  which  are  univer- 
sally received;  but  where  demonstrations  become  in- 
dispensably necessary,  they  are  introduced  with  as 
much  brevity  and  perspicuity  as  the  subject  would 
admit. 

In  short,  every  endeavour  has  been  exerted  to  make 
the  work  correspond  with  the  intention ;  which  was  to 
produce  a  cheap  and  comprehensive  abridgment  of 
Natural  Philosophy  adapted  to  the  understanding  of 
the  generality  of  persons,  and  it  is  now  left  with  a 
liberal  public  to  determine  on  the  utility  and  success 
of  the  undertakhig. 


CONTENTS. 


The  Laws  of  Motion 13 

Attraction       16 

Attraction  of  Cohesion ib. 

Electrical  Attraction 21 

Magnetical  Attraction 22 

To  make  artificial  Magnets 25 

Mechanics 27 

Gravitation      .     .     .  ^ ib. 

Collision  of  Elastic  and  Nonelastic  Bodies 31 

Pendulums 34 

Centre  of  Gravity 36 

Mechanic  Powers 39 

The  Lever ib. 

Wheel  and' Axis- " 43 

The  Pulley  ^ 45 

The  Wedge 48 

The  Inclined  Plane 49 

The  Screw'' 51 

Friction  or  Attrition 52 

Pneumatics 55 

Weight  and  Pressure  of  Air ib. 

The  Elasticity  of  the  Air  and  the  Weight  and  Density  of 

the  Atmosphere 58 

Air  Pump        ^ 61 

Experiments  on  the  Pressure  of  Air 63 

Experiments  on  the  Elasticity  of  Air 67 

Miscellaneous  Experiments 70 

Barometer* 73 

Diagonal  Barometer ' 76 

Pressure  of  the  Atmosphere  according  to  the  Barometer  .  ib. 

Thermometer 78 

b 


X  CONTENTS. 

Page 

Hygrometer 80 

Air  Gun « 81 

Diving  Bell ib. 

Sound 84 

The  Vibration  of  extended  Strings 86 

Musical  Sounds 89 

Speaking  Trumpets 90 

Echoes 91 

Hydrostatics 93 

Hydrostatic  Principles 94 

On  the  Specific  Gravity  and  Density  of  Bodies     ....  101 
Principles  and  Experiments  demonstrating  the  Density  and 

Specific  Gravity  of  Bodies 102 

To  determine  the  Specific  Gravity  of  Bodies 108 

The  Hydrometer Ill 

Table  of  Specific  Gravities 113 

Hydraulics 114 

Siphon  or  Crane 115 

Natural  and  Artificial  Fountains 1 1 7 

The  common  Lifting  Pump 120 

The  Forcing  Pump 124 

De  la  Hire*s  Pump 125 

Hair  Rope  Pump 126 

Archimedes' Screw  for  raising  Water 127 

Steam  Engine 128 

Electricity 131 

Electrical  Machine 137 

Positive  and  Negative  Electricity 140 

Points 142 

Electrical  Attraction  and  Repulsion 143 

Leyden  Phial 145 

Electric  Battery 151 

Meteorology 152 

Rain ib. 

Clouds ^    .     .  155 

Hail 158 

Lightning  and  Thunder ib. 

Wind 161 

Light  and  Colours 166 


CONTENTS. 


X 


Page 

Prism .  169 

Optics .  173 

Refracted  Vision 174 

Reflected  Vision 179 

Lenses 182 

Camera  Obscura 186 

Magic  Lantern 187 

Burning  Glass ib. 

Telescopes 188 

The  Astronomical  Telescope 189 

The  Land  Telescope ISO 

Reflecting  Telescopes 191 

The  Solar  System 194. 

The  Copernican  System 196 

The  Sun  and  Planets 200 

The  Earth  distinctly  considered 205 

Motion  of  the  Earth 207 

Changes  of  the  Seasons 210 

The  Moon's  Motion 216 

Phases  of  the  Moon 218 

Eclipses  of  the  Moon 220 

Eclipses  of  the  Sun 224 

Explanation  of  Terms 226 


ELEMENTS 


OF 


NATURAL  PHILOSOPHY, 


Natural  philosophy  is  that  science  which 
considers  the  powers  of  nature,  the  properties  of 
natural  bodies,  and  their  action  on  each  other. 

The  following  are  the  three  general  laws,  by  which 
the  motions  of  natural  bodies  are  governed. 

First :  Every  body  continues  in  a  state  of  rest,  or 
moves  uniformly  in  a  right  line,  unless  it  be  com- 
pelled to  change  that  state  by  the  action  of  some  ex- 
ternal force. 

Thus  a  ball  discharged  from  a  cannon  would  per- 
severe in  its  motion  for  ever,  if  it  were  not  retarded  by 
the  resistance  of  the  atmosphere  and  the  operation  of 
gravity.  Or  a  top  put  in  motion  would  have  an  end- 
less revolution,  if  it  were  not  impeded  by  the  air  and 
the  friction  produced  by  its  point  on  the  plane  on 
which  it  moves.  According  to  this  law  the  heavenly 
bodies  also  preserve  their  progressive  motions  undi- 
minished in  those  regions,  which  are  void  of  all  re- 
sistance. 

Secondly :  The  change  of  motion  is  always  propor- 
tional to  the  moving  force  by  which  it  is  produced, 
and  it  is  made  in  the  line  of  direction  in  which  that 
force  is  impressed.  For,  by  the  first  law,  motion  can- 
not be  generated  in  a  body  without  some  external 
impulse;  and,  as  the  motion  thus  generated  is  direct- 

A 


14 


Laws  of  Motion, 


ed  in  a  right  line  with  a  velocity  equal  to  the  degree 
of  impulse,  the  course  of  a  body  in  motion  can  only 
be  altered  by  a  fresh  impulse,  and  is  then  compound- 
ed of  its  own  velocity  and  the  impelling  force ;  that 
is,  the  body  will  be  either  accelerated  or  retarded  in 
a  right-lined  direction  in  proportion  to  the  compound 
force  of  the  two  impressions. 

In  the  square  a  b  c  d,  if  a  body  be  put  in  motion 
by  an  impelling  force  in  the  di-  a  b 

rection  a  b,  and  if  at  the  same  in- 
stant it  be  acted  upon  in  an  equal 
degree  by  another  impulsion  in 
the  line  a  c,  the  body  will  be  pro- 
jected with  a  force  and  direction, 
compounded  of  the  two  impulses, 
which  is  represented  by  the  line 
a  D.  In  like  manner,  if  a  ship  at  sea  sail  before  the 
wind  in  the  line  e  f,  due  £ 
east,  at  the  rate  of  eight 
miles  an  hour,  and  a  current 
set  from  the  north,  in  the 
direction  e  g  ,  at  the  rate  of 
four  miles  an  hour;  the  ves- 
sel will  be  driven  between  ^ 
the  north  and  the  east  in  the  direction  e 
pounded  of  the  two  acting  forces,  at  the  rate  of  nine 
miles  an  hour  nearly. 

Thiraiy :  Action  and  reaction  are  always  equal  and 
contrary.  Or  the  action  of  two  bodies  on  each  other 
is  always  equal,  but  in  contrary  directions. 

Thus,  if  a  stone  be  pressed  by  the  hand,  the  reac- 
tion or  compressive  force  of  the  stone  on  the  hand  is 
equal  to  the  pressure  upon  the  stone.  Or  when  a  horse 
draws  a  load,  the  power  of  the  horse  is  diminished,  or 
the  animal  is  drawn  back,  with  a  force  equal  to  that 
which  puts  the  load  in  motion :  for,  if  the  weight  of 
the  load  be  increased  till  it  is  equal  to  the  strength  of 
the  horse,  it  will  remain  at  rest,  although  the  whole 


Laxvs  of  Motion.  15 

force  of  the  atiimal  be  in  action.  If  a  loadstone  and  a 
piece  of  iron  of  equal  weight  be  suspended  by  strings 
near  each  other,  the  mutual  force  or  attraction  be- 
tween them  will  cause  an  equal  action,  and  the  two 
bodies  will  leave  their  respective  positions  with  an 
equal  impulse  and  velocity,  and  meet  in  a  point  equally 
distant  from  each. 

If  the  bodies  be  unequal  they  will  meet  in  a  point 
whose  distance  from  the  bodies  will  be  reciprocally 
proportional  to  the  difference  of  the  powers. 

Lastly :  If  two  floating  vessels  of  equal  magnitude 
be  attached  by  a  rope  at  some  distance  from  each 
other,  a  force  applied  to  the  rope  in  either  vessel  will 
mutually  draw  them  together,  with  an  equal  velocity, 
till  they  meet  in  a  point  equidistant  from  their  first 
position. 


16 


ATTRACTION. 

Attraction  is  a  general  expression  used  to  de- 
note the  cause,  power,  or  principle,  by  which  all  bodies 
mutually  tend  towards  each  other. 

This  universal  principle  is  considered  as  one  of  the 
first  agents  of  nature  in  all  her  operations.  By  this 
extraordinary  power  the  minutest  particles  of  matter 
cohere,  bodies  are  formed,  and  even  the  whole  uni- 
verse is  governed  by  its  influence ;  yet,  after  endless 
opinions,  its  cause  is  still  concealed  in  the  bosom  of 
nature.  We  clearly  view  the  effects  of  attraction,  and 
decide  on  its  laws,  but  human  ingenuity  has  not  been 
■able  to  fathom  its  principle  or  essence. 

Newton  considers  it  as  a  power  or  virtue  proceed- 
ing from  bodies  in  every  direction,  which  decreases 
in  energy  or  effect  in  proportion  as  the  squares  of  the 
distance  from  the  body  increase ;  that  is,  at  any  given 
distance  it  will  be  four  times  as  great  as  at  twice  that 
distance,  and  nine  times  as  great  as  at  thrice  the  dis- 
tance; and  so  on  in  like  proportion. 

This  law,  however,  only  relates  to  one  branch  of 
attraction,  as  it  has  different  modifications  in  its  dif- 
ferent divisions :  these  are  called  attraction  of  cohe- 
sion, electrical  attraction,  magnetical  attraction,  and 
attraction  of  gravitation. 

Attractio7i  of  Cohesion, 

Attraction  of  cohesion  is  that  force  by  which 
the  particles  of  bodies  mutually  tend  towards  each 
other.  It  is  the  most  powerful  in  the  point  of  contact, 
or  where  the  particles  touch ;  at  a  little  distance  it  be- 
comes considerably  less ;  and  when  the  particles  arc 
still  further  removed,  the  effect  is  rendered  insensible. 

The  power  of  corpuscular  attraction,  or  the  cohe- 
sion of  particles  of  small  bodies,  may  be  shown  by  a 
variety  of  amusing  experiments. 


Attraction  of  Cohesion,  17 

Take  two  leaden  bullets,  with  apart  cutaway  from 
each  of  their  surfaces,  so  as  to  form  a  small  plane, 
perfectly  smooth  and  even.  This  being  done,  press 
the  flat  surfaces  together,  twisting  the  bullets  with  the 
lingers  as  they  are  pressed;  then  the  parts  which 
touch  each  other  will  adhere  or  be  attracted  with  such 
force,  as  to  require  a  power  of  more  than  fifty  pounds 
weight  to  separate  them. 

The  twisting  of  the  planes  of  the  bullets  serves 
only  to  bring  the  parts  nearer  together;  for,  as  it  is 
scarcely  possible  to  cut  the  surfaces  perfectly  even, 
this  twisting  pressure  tends,  from  the  softness  of  the 
metal,  to  rub  down  the  inequalities,  to  expel  the  air 
which  is  contained  between  the  planes,  and  to  bring  a 
greater  number  of  parts  into  contact. 

As  the  formation  of  bodies  arises  from  the  ad- 
hesion or  attraction  of  the  particles  of  matter;  if  the 
metal  in  the  above  experiment  were  perfectly  free 
from  porosity,  and  the  planes  mathematically  even,  on 
joining  them  together,  the  parts  in  adhesion  would 
be  as  firm  and  inseparable  as  any  other  parts  of  the 
bodies. 

But  as  corpuscular  attraction  extends  only  to  infi- 
Xiitely  small  distances,  and  as  a  considerable  part  of 
the  surfaces  cannot  come  into  contact,  not  only  from 
the  porosity  of  the  metal,  but  from  the  inequalities  of 
©f  the  planes;  the  elasticity  of  the  air,  which  is  con- 
tained in  the  interstices,  is  perpetually  endeavouring 
to  force  them  asunder. 

The  planes  can  only  adhere  when  the  power  of  the 
parts  in  contact  is  greater  than  the  natural  gravity,  and 
the  elastic  pov/er  of  the  air  contained  between  them ; 
therefore  the  cohesive  force  is  proportionable  to  the 
number  of  parts  that  touch  each  other. 

If  oil,  tallow,  or  any  other  unctuous  body,  be  smear- 
ed on  the  surface  of  the  planes,  in  such  a  manner  as  to 
cxchide  a  principal  portion  of  the  air  which  is  contain- 
ed between  them,  the  planes  will  adhere  with  much 


18  Attraction  of  Cohesion, 

greater  firmness ;  so  that  plates  of  brass,  silver,  or 
iron,  of  small  dimensions,  may  be  made  to  cohere  with 
such  force,  as  would  require  the  united  power  of  a 
number  of  men  to  pull  them  asunder.  Experiment 
has  shown,  that  plates  not  more  than  two  inches  in  di- 
ameter have  taken  a  force  of  950ll)s.  weight  to  sepa- 
rate them,  when  the  surfaces  have  been  heated  and 
smeared  with  boiling  grease,  and  then  left  to  cool  be- 
fore the  power  was  applied.  This  adhesive  power  or 
quality  in  the  particles  of  bodies,  is  not  occasioned  or 
aided  by  the  gravitating  weight  of  the  atmosphere ; 
for  it  is  found  by  experiment,  that  it  requires  the  same 
weight  to  separate  them  whether  they  be  joined  to- 
gether in  the  open  air  or  in  vacuo. 

Take  two  plates  of  glass  ground  even,  and  place 
them  edgewise,  very  near  to  each  other,  in  a  vessel 
of  water;  first  wetting  the  insides  of  the  plates;  then 
the  attracting  power  of  the  glass  will  raise  the  water 
which  is  contained  between  them  considerably  above 
the  general  surface  of  the  fluid.  This  height  is  pro- 
portional to  the  distance  of  the  plates  from  each  other; 
if  they  be  placed  about  the  hundredth  part  of  an  inch 
asunder,  the  water  will  rise  more  than  an  inch  above 
the  common  surface  in  the  vessel. 

The  water  ascends  by  the  attraction  of  the  plates, 
till  the  gravity  or  weight  of  the  ascending  fluid  is 
equal  to  the  power  of  attraction  between  the  plates ; 
and  as  the  power  of  the  glass  by  which  the  water  is  at- 
tracted is  always  the  same,  it  is  evident  that,  as  the 
sides  approach,  the  gravity  of  the  fluid  at  equal  heights 
becomes  less,  and  consequently  the  elevation  will  be 
greater,  before  the  weight  of  the  water,  or  power  of 
gravitation,  becomes  equal  to  the  attractive  force.  If 
the  plates  be  placed  angularly,  or  touch  each  other  at 
one  of  the  ends,  the  water  will  rise  in  the  form  of  a 
hyperbolic  curve. 

Take  some  small  pieces  of  cork  about  the  size  of  a 
pea,  and  place  them  on  the  surface  of  a  vessel  of  water, 


Attraction  of  Cohesion,  19 

and  move  them  very  gently  towards  each  other;  then, 
when  the  balls  approach  the  sphere  of  each  other's 
attraction,  their  motion  will  be  instantly  accelerated, 
and  the  bodies  will  mutually  come  into  contact.  In 
like  manner,  if  the  balls  be  placed  at  a  small  distance 
from  the  edge  of  the  vessel,  they  will  immediately 
move  towards  it.*  Rain,  in  falling  from  the  clouds 
through  the  atmosphere,  is  divided  into  parts,  which 
are  formed  into  small  spheres  by  the  mutual  attrac- 
tion of  the  particles  that  compose  them.  The  drops  of 
rain  which  rest  upon  cabbage  leaves,  and  other  vege- 
tables that  are  covered  with  a  fine  powder,  also  assume 
a  spherical  appearance  from  the  same  principle.  Glo- 
bules of  quicksilver  are  formed  in  like  manner,  by  the 
attraction  of  their  parts,  and  incorporate  by  the  same 
principle  when  difi'erent  globules  come  into  contact. 
If  a  piece  of  board  or  any  other  plane  be  laid  on  the 
surface  of  water,  it  will  require  a  power  six  times  as 
great  as  the  weight  of  the  body  to  take  it  up  perpen- 
dicularly. 

These  and  many  other  facts,  which  daily  occur  in 
the  common  occupations  of  life,  serve  to  show  the 
universal  tendency  of  that  corpuscular  attraction 
which  exists  between  small  bodies;  whilst  the  attrac- 
tion of  gravitation,  extending  to  indefinite  distances, 
causes  all  the  regular  changes  and  successions  in  the 
planetary  system.  Thus  the  divine  Being,  by  a  dif- 
ferent modification  of  the  same  incomprehensible  prin- 
ciple, compounds  and  preserves  the  whole  system  of 
his  works. 

^  *  The  above  phenomena  are  not  produced  by  the  mutual  attrac- 
tion between  the  balls  and  the  edp;e  of  the  vessel,  or  between  the 
balls  themselves.  This  will  be  evident  by  suspending  them,  or  any 
other  bodies,  by  lines  or  threads;  for,  thoui^h  brought  ever  so  near 
to  each  other,  no  sensible  attraction  will  then  take  place.  This  ap- 
parent attraction  is  occasioned  by  the  water  rising  round  the  ball"? 
and  the  edge  of  the  vessel  above  the  common  surface,  and  the  ten- 
dency of  light  floating  bodies,  from  the  laws  of  specific  gravity,  to 
move  to  the  highest  part  of  the  fluid  in  which  they  float.    Ed. 


2p  Attraction  of  Cohesion. 

A  species  of  corpuscular  adhesion,  dlled  capillary 
attraction,  is  a  striking  proof  of  his  goodness,  and 
tends  to  produce  those  blessings  of  nature  which  we 
enjoy. 

Capillary  attraction  is  a  term  used  to  denote  the  as- 
cent of  fluids  through  those  small  pipes  or  tubes,  that 
compose  a  considerable  part  of  the  animal  as  well  as 
vegetable  body.  By  these  tubes,  which  are  as  various 
in  their  number  as  they  are  different  in  capacity,  na- 
ture conveys  nutriment  to  supply  the  most  distant 
branches  of  vegetation,  where  it  could  never  arrive  by 
the  ordinary  motion  of  fluids. 

These  tubes  attract  in  an  inverse  proportion  to  their 
diameters,  as  the  glass  plates  attract  in  proportion  to 
their  contiguity:  that  is,  those  tubes  which  are  the 
smallest,  raise  the  fluid  to  the  greatest  height,  and  the 
larger  to  a  less  height  in  a  reciprocal  proportion. 

When  the  earth  receives  rain  on  its  surface,  the 
fluid  is  attracted  through  all  the  internal  and  contigu- 
ous parts ;  it  is  then  absorbed  by  the  roots  of  trees, 
plants.  Sec;  and  afterwards  carried  by  capillary  attrac- 
tion to  the  most  extended  ramifications,  through  the 
multitudinous  pores  contained  in  the  trunk  and  its 
branches.* 

Melted  tallow  and  oil  supply  the  flame  of  candles 
and  lamps  by  capillary  attraction.  Water  poured 
round  the  bottom  of  a  heap  of  sand,  sugar,  ashes,  or 
any  other  porous  substance,  will  diffuse  itself  till  it  has 
reached  the  summit.  This  attracting  power  is  likewise 
observable  in  lump  sugar,  sponge,  linen,  and  many 
other  bodies,  when  their  lower  extremities  are  dipped 
into  water.  In  short,  every  porous  or  capillary  sub- 
stance is  a  conductor  for  the  attracted  fluid.  This  at- 
tracting power  acts  independently  of  atmospheric 
pressure;  for  if  capillary  tubes  be  placed  in  a  vessel  of 

*  Capillary  attraction  is  not  of  itself  adequate  to  this  end;  it  can 
only  be  rendered  effectual  by  the  principle  of  vegetable  life.    £d. 


Mlectrical  Attraction.  21 

water  under  an  exhausted  receiver,  the  fluid  will  as- 
cend to  the  same  height,  as  when  the  experiment  is 
made  in  the  open  air. 

As  a  curious  instance  of  the  attraction  of  fluids, 
throughthe  pores  of  the  skin;  sailors,  left  without  fresh 
water,  frequently  dip  their  clothes  in  the  sea,  and  ap- 
ply them  wet  to  their  bodies,  which  then  attract  the 
pure  particles  of  the  fluid,  and  by  this  mean  they  allay 
the  extremity  of  thirst. 

After  cohesive  and  capillary  attraction,  the  next  divi- 
sion of  this  extraordinary  power  will  lead  us  to  take  a 
slight  view  of  electrical  attraction ;  but  as  it  will  be 
necessary  hereafter  to  enter  into  a  more  general  detail 
of  electricity,  it  will  be  sufl[icient,  for  the  sake  of  order, 
to  give  here  only  some  account  of  the  leading  princi- 
pies  of  this  subject. 

Electrical  Attraction, 

By  electrical  attraction  is  meant  that  power  which 
is  excited  by  heat  and  friction,  in  glass,  amber,  and 
resinous  bodies,  such  as  sealing-wax,  resin,  &c. 

If  a  cylindrical  glass  tube  be  heated  by  rubbing  it 
briskly  with  an  old  silk  handkerchief,  and  held  at  a 
certain  distance  from  the  downy  part  of  a  feather,  or 
pieces  of  gold  or  brass  leaf,  some  of  those  pieces  which 
come  within  the  atmosphere  of  the  excited  attraction 
will  immediately  fly  towards  the  tube,  and  rest  upon 
it,  while  others  are  repulsed  with  a  contrary  effect. 

If  a  glass  globe  have  its  axis  placed  horizontally,  and 
some  small  flaxen  threads  be  suspended  from  a  semi- 
circular wire,  fastened  over  the  upper  part  of  its  surface; 
when  the  globe  is  at  rest,  these  threads  will  hang  per- 
pendicular and  parallel  to  each  other,  according  to 
their  respective  gravitations;  but  on  turning  the  globe 
briskly  its  rotatory  motion  will  communicate  the  same 
motion  to  the  air  that  surrounds  it,  and  this  will  impel 

B 


22  Magnetical  Attraction. 

or  turn  up  the  ends  of  the  threads,  in  the  direction  of 
the  current  of  air  which  is  produced  by  the  motion  of 
the  globe :  but  on  applying  a  dry  hand  or  rubber  to 
the  surface  of  the  sphere,  still  continuing  its  motion, 
an  electrical  attraction  will  be  excited,  which  will 
straighten  the  threads,  draw  them  from  their  parallel 
position,  and  converge  them  in  lines  tending  directly 
towards  the  centre  of  the  globe. 

The  attraction  of  sealing-wax  and  other  resinous 
substances,  which  is  excited  by  rubbing  them  with  a 
piece  of  woollen  cloth  or  on  the  coat  sleeve,  is  too  ge- 
nerally known  to  need  any  observation  here.  Amber 
has  this  peculiar  quality  of  attracting  light  substances, 
which  was  the  only  instance  of  electrical  attraction 
that  was  known  for  ages. 

Magnetical  Attraction. 

This  species  of  attraction  is  a  surprising  instance 
of  that  strong  and  subtile  power,  which  is  exerted  be- 
tween iron  and  the  loadstone,  and  varies  in  proportion 
to  the  distance  of  the  bodies  from  each  other.  This 
singular  stone  not  only  possesses  an  attractive  but  like- 
wise a  repelling  power.  Some  suppose  that  it  derives 
these  qualities  from  the  position  in  which  it  lies  in  the 
earth;  others,  that  the  earth  itself  is  a  magnet,  from 
which  all  others  derive  their  power,  and  that  there  is 
a  great  similarity  between  the  electrical  and  magneti- 
cal fluid ;  for  if  a  small  bar  of  iron  be  balanced  on  a 
fine  point,  on  applying  the  magnet,  one  end  will  at- 
tract and  the  other  repel  the  iron.*  From  this  it  is 
presumed,  that  the  magnetical  power  passes  like  an 
electrical  current,  and  that  it  has  its  positive  and  ne- 

*  Both  ends  of  the  magnet  will  attract  a  piece  of  iron  or  unmag- 
netised  steel ;  magnetic  repulsion  takes  place  only  between  the 
like  poles  of  two  magnets.  £d. 


Magnetical  Attraction.  23 

gative  ends,  like  the  opposite  sides  of  an  electrical 
cylinder  in  motion. 

If  a  bar  of  iron,  or  a  common  poker,  be  set  nearly- 
upright,  with  one  end  on  the  ground,  it  will,  in  a  short 
time,  attract  an  unmagnetised  needle  with  its  upper 
end;  but  on  turning  the  other  e'xtremity,  the  needle 
will  recede.*  From  this  peculiar  effect  it  is  supposed, 
that  the  earth  emits  a  magnetical  fluid,  and  that  a  part 
of  it  is  retained  in  passing  through  the  bar  or  metallic 
conductor. 

The  course  of  the  magnetical  effluvia  may  be  made 
visible  in  the  following  manner.  Lay  a  sheet  of  white 
paper  over  a  bar  magnet,  and  sift  some  fine  steel  filings 
upon  it;  these,  in  falling  upon  the  paper,  will  arrange 
themselves  in  the  magnetical  course,  and  form  curved 
lines  round  both  sides  of  the  magnet,  crossing  each 
other  at  the  two  extremities,  or  in  the  respective  poles 
of  the  magnet. 

Take  a  needle,  which  is  used  for  the  compass,  be- 
fore it  is  magnetised,  and  balance  it  horizontally  on  a 
fine  point;  then  take  it  off  and  communicate  the  at- 
traction either  by  a  natural  or  artificial  magnet,  and 
place  it  again  on  the  point :  it  will  now  lose  its  hori- 
zontal position,  and  one  end  of  the  needle  will  dip  or 
sink  downwards,  making  an  angle  of  about  73  degrees 
with  the  surface  of  the  earth.  This  is  what  is  called 
the  dip  of  the  needle,  and  is  supposed  to  arise  from 
the  subtile  power  that  issues  from  the  earth  in  an 
oblique  direction,  and  which  passes  through  the  nee- 
dle in  its  meignetical  course.  In  further  support  of  this 
opinion,  wis  find  that  the  pole  of  a  magnetical  bar  and 
the  same  pole  of  the  needle  repel  each  other;  whilst 
the  opposite  poles,  or  those  that  give  and  receive  the 
magnetical  fluid,  mutually  attract. 

Natural  magnets  generally  attract  each  other  with 
less  force  than  those  which  are  made  of  steel.    Rust 

*  Vide  p.  22,  note. 


24  MagtiPtical  J  ttr  action, 

and  fire  greatly  injure,  or  totally  destroy,  the  magne- 
tical  power.  Many  substances  beside  iron  and  steel 
are  also,  in  some  degree,  affected  by  the  magnet;  but 
this  is  supposed  to  arise  from  the  ferruginous  matter 
they  contain. 

When  magnets  of  different  sizes  are  opposed  to  each 
other,  they  mutually  repel*  at  a  small  distance,  but 
adhere  if  they  be  brought  into  contact;  for  in  this  case 
the  power  of  the  superior  magnet  is  capable  of  repel- 
ling that  of  the  inferior,  and  of  changing  its  poles, 
which  then  causes  them  to  attract  one  another. 

To  show  the  attractive  power  of  a  good  magnet, 
let  it  be  suspended  from  one  end  of  a  scale  beam,  and 
counterpoised  by  weights  at  the  other;  then  fix  a 
piece  of  flat  iron  about  |  of  an  inch  from  the  bottom 
of  the  magnet  and  it  will  then  descend  and  adhere  to 
the  iron :  if  they  be  again  separated,  and  4|  grains  be 
added  to  the  opposite  end  of  the  beam,  the  weight  will 
exactly  balance  the  attractive  power  of  the  magnet, 
and  oppose  its  descent;  but  if  any  part  of  the  weight 
be  taken  away,  the  magnet  will  preponderate  and  de- 
scend as  before.  If  it  be  placed  at  half  the  above  dis- 
tance, it  will  take  four  times  the  former  weight,  or 
about  171  grains  to  balance  the  scale  beam;  conse- 
quently the  attractive  force  of  the  magnet,  at  the  single 
distance  from  the  iron,  is  to  its  force  at  double  the  dis- 
tance as  4  to  1 ;  that  is,  reciprocally  as  the  squares  of 
the  distances. 

The  power  of  the  smaller  magnets  is  generally 
greater,  in  proportion  to  their  weight,  than  that  of  the 
larger,  for  large  magnets  will  seldom  take  up  more 
than  three  or  four  times  their  weight;  but  the  smaller 
will  suspend  ten  or  twelve,  and,  in  some  instances, 
twenty-four  times  their  own  weight. 


*  The  like  poles  only  mutually  repel ;  the  contrary  poles  always 
mutually  attract.  JF.d. 


25 


To  make  Artificial  Magnets. 

Among  the  various  modes  that  are  recommended, 
by  diiFereiit  persons,  the  following  appears  the  most 
simple,  and  yet  sufficiently  powerful,  for  any  common 
purpose. 

Place  two  magnets,  a  and  b,  in  a  right  line;  laying 
the  north  end  of  one     ({jv, 
towards  the  south  end 
of  the  other:  (by  the 
north  or  south  end  of 
the  magnet,  is  meant,  ^' — 3  "X" 

that  end  which  would  point  towards  the  north  or  south 
pole,  if  the  bar  Avere  balanced  on  a  point;  the  north 
end  is  generally  marked  by  a  fine  line  cut  across  near 
the  extremity  of  the  bar.)  After  the  magnets  are  thus 
placed,  let  the  bar  c,*  which  is  to  receive  the  magne- 
tic power,  rest  upon  the  two  extremities  of  the  former, 
placing  the  end  that  is  intended  for  the  north,  upon 
the  south  end  of  the  magnet  that  supports  it:  this  be- 
ing done,  take  two  other  magnetical  bars,  d  and  e, 
and  bring  the  north  and  south  ends  of  them  together 
upon  the  middle  of  the  bar  c ,  raising  up  the  opposite 
end,  till  the  angles  which  are  formed  on  the  bar  be- 
come equal;  then  separate  e  and  d,  by  drawing  them 
diiferent  ways  to  each  end  of  c,  still  keeping  the  ex- 
tremes of  D  E  at  equal  distances  from  the  plane:  after 
they  are  drawn  off,  join  them  together  about  a  foot 
above  the  plane  c,  and  again  place  then  in  contact  on 
the  middle  of  the  bar;  after  this  has  been  repeated  four 
or  five  times,  turn  each  of  the  other  three  sides  up- 
ward and  pursue  the  same  operation,  which  will  com- 
municate a  strong  and  permanent  magnetism  to  the 
bar  of  steel. 

*  This  bar  must  be  of  hard  or  highly  tempered  steel;  for  neither 
iron  nor  soft  steel  will  acquire  i\ny  permanent  magnetism.  Ed. 


26  To  make  Artificial  Magnets. 

The  magnetic  power  may  be  easily  communicated^ 
by  what  is  called  the  horse- shoe  magnet,  from  its  re- 
semblance to  that  form.  Lay  the  bar  to  be  magnetised 
upon  the  extremes  of  two  magnets  as  in  the  preceding 
directions,  and  place  the  two  ends  of  the  horse-shoe 
magnet  upon  the  middle  of  the  bar,  observing  that  the 
north  pole  of  the  magnet  is  placed  towards  that  which 
is  intended  for  the  south  pole  in  the  bar;  then  draw  it 
backwards  and  forwards  live  or  six  times,  and  after- 
wards take  off*  the  magnet  from  the  middle  of  the  bar. 
The. same  mode  must  be  followed  on  each  of  the  three 
other  sides. 


MECHANCIS. 


Gravitation, 


Gravitation  forms  the  last  division  of  attraction, 
and  is  here  applied  to  that  force  with  which  bodies  tend 
towards  the  centre  of  the  earth,  or  by  which  they  fall 
perpendicularly  to  its  surface :  but  as  this  property  is 
the  vital  principle  of  mechanics,  it  is  placed  here  as  an 
introduction  to  the  mechanic  powers. 

Gravitation  seems  to  differ  from  corpuscular  at- 
traction only  as  a  part  differs  from  a  whole:  the  attrac- 
tive power  which  singly  unites  the  particles  of  smaller 
bodies,  may  form  that  gravitating  power,  in  the  aggre- 
gate, which  governs  the  system  of  the  universe;  thus, 
considering  the  attractive  influence  of  bodies  as  pro- 
portional to  their  magnitudes,  the  less  will  be  govern- 
ed by  the  greater,  and  those  which  are  on,  or  near,  the 
surface  of  the  earth  will  tend  towards  its  centre: 
Bodies  not  only  gravitate  towards  the  earth,  but  like- 
wise towards  great  elevations  or  mountains  on  differ- 
ent parts  of  its  surface;  for  if  a  ball  be  suspended  by  a 
line,  and  placed  on  different  sides  of  a  high  mountain, 
it  will  gravitate  on  every  side  towards  the  mountain. 

The  power  of  gravity  is  the  same  in  all  bodies; 
therefore,  taking  away  the  resistance  of  the  air  or  the 
medium  through  which  they  fall,  the  descent  of  all 
bodies  from  the  same  height  will  be  performed  in  the 
same  time,  whether  they  be  great  or  small,  light  or 
heavy.  For  example ;  if  a  piece  of  gold  and  a  feather, 
or  any  other  bodies  the  specific  gravities  of  which  are 
different,  be  dropped  from  a  given  height,  through 
the  atmosphere,  the  superior  gravity  of  the  gold  will 
more  effectually  overcome  the  resistance  of  the  air 
than  the  inferior  weight  of  the  feather,  and  consequent- 
ly it  will  fall  much  sooner  to  the  ground ;  but  if  they 


\ 


28 


Gravitation, 


both  fall,  at  the  same  instant,  from  the  slip  of  an  ex» 
haasted  receiver,  they  will  arrive  at  the  bottom  in 
equal  times;  for  as  the  resisting  medium  of  the  air 
is  here  taken  away,  the  bodies  descend  with  equal 
velocities. 

Falling  bodies  gravitate  with  an  increasing  veloci- 
ty as  they  approach  the  surface  of  the  earth;  this  ac- 
celerated motion  is  produced  by  the  constant  power 
of  gravity,  which,  by  adding  a  fresh  impulse  at  every 
instant,  gives  an  additional  velocity  and  an  increasing 
motion  in  every  moment  of  time.  The  space  through 
which  a  body  falls  by  the  power  of  gravity  in  the  lati- 
tude of  London  is  16t2  feet,  in  the  first  second  of  time, 
four  times  that  distance  in  two  seconds,  nine  times  in 
three  seconds,  and  sixteen  times  in  four  secoad^;  in- 
creasing in  velocity  according  to  the  squares  of  the 
distance  through  which  the  body  descends. 

Let  A  B  represent  the  equal  parts  of  time  through 
which  a  body  is  accelerated  in  fall- 
ing; then  the  velocity  acquired  in 
passing  through  each  space  by  the 
continual  impulse  of  gravity,  is  re- 
latively as  the  lengths  of  the  paral- 
lels a  I,  b2,  c  3,  &:c.,  throughout 
the  whole  figure;  that  is,  the  velo- 
city increases  as  the  lines  increase 
in  length,  and  the  quantity  of  the 
velocity-^  is  equal  to  the  square  of 
the  time;  for  if  5  e*  represent  the 
velocity  acquired  in  falling  through 
the  space  a  5,  the  sum  of  the  veloci- 
ties,* or  all  the  similar  triangles  taken  together,  which 
are  contained  in  a  5  ^  will  amount  to  25,  which  is 
equal  to  the  square  of  the  time  a  5 ;  and  so  on  of  any 
other  point  of  time. 

*  That  is,  the  whole  space  descended  through,  will  be  propor- 
tional to  the  square  of  the  time  of  descent.  £d, 


Gravitation.  29 

The  spaces  described  by  a  uniform  motion  with  the 
last  acquired  velocity,  during  a  time  equal  to  that  of 
the  acceleration  from  the  beginning,  will  be  double 
the  space  described  by  the  accelerated  motion.  For  if 
B  D  be  equal  in  time  to  a  b,  and  b  c  and  its  parallels 
express  the  equable  time  of  motion,  the  parallels  like- 
wise express  the  velocities  as  in  the  preceding  case ; 
but  they  are  double  the  spaces  of  the  accelerated 
motion. 

The  law  of  acceleration  in  bodies  descending  per- 
pendicularly, holds  equally  in  point  of  time  with  those 
bodies  that  are  projected.  For  if  a  stone  be  dropped 
from  the  top  of  a  tower,  and  another  of  the  same 
weight  be  thrown  horizontally  at  the  same  instant, 
the  two  different  bodies  will  reach  the  ground  at 
the  same  moment  of  time.  Even  if  a  ball  were  fired 
with  any  force  horizontally  from  a  cannon,  on  the  top 
of  a  tower,  it  would  describe  the  line  of  its  course  in 
the  same  time  that  another  ball  would  fall  perpendi- 
cularly from  the  top  to  the  bottom,  supposing  them  to 
meet  with  no  resistance  from  the  air. 

For  if  A  D  be  the  horizontal  line  of  direction,  which 
the  ball  w^ould  move  in  by  ^t^.. 
the  force  of  the  gunpowder,  ^ 
and  A    B  the  perpendicular 
line  of  gravitation,  then,  ac- 
cording to  the  second  law  of 
motion,  the  ball  will  not  pass 
in  either  direction,  but  in  the 
line   A    c,    compounded    of 
them  both:    again,    by  the 

same    law,    it    would    pass  ^*^  *" ^c 

through  the  curve  a  ^,  in  the  same  time  that  its  gra- 
vity alone  would  carry  it  from  a,  to  a  1;  therefore 
the  time  of  describing  a  a  is  equal  to  the  time  of 
describing  a  1 :  a  Z*,  to  a  2:  a  c?,  to  a  3 ;  and  so  on 
for  the  whole,  making  the  sum  of  the  times  of  the 

C       - 


%. 


■^. 


i  \?/» 


\ 

\ 


30  Gravitation, 

direction  of  the  ball  a  c,  equal  to  the  times  of  the 
gravity  a  b. 

If  a  ball  be  dropped  from  the  topmast  of  a  ship, 
even  supposing  the  vessel  to  sail  through  the  water 
with  a  velocity  of  ten  miles  an  hour,  it  will  fall  exactly 
at  the  bottom  of  the  mast.  For  let  a  c  be  the  ship's 
mast,  and  its  position  when  the  ball  a 
is  dropped  from  a  ;  c  d  the  distance 
sailed  during  the  time  of  the  de- 
scent; and  B  D  the  second  situation, 
when  the  ball  strikes  the  deck. 
Then  the  force  or  projecting  veloci- 
ty of  the  vessel  c  diha  b  carries  it 
towards  b,  and  the  force  of  gravity 
acts  from  a  towards  c ;  but  the  com-  _ 

pound  force  carries  the  ball  to  d,  in    c  d 

the  direction  a  d,  and  in  the  same  time  that  it  would 
fall  from  a  to  c,  if  the  vessel  were  at  rest.  And  as  the 
velocity  of  the  ball  and  vessel  are  equal,  it  apparently 
drops  perpendicularly  down  the  side  of  the  mast,  as 
it  describes  the  curve  of  projection;  but  a  person  in 
a  vessel  at  anchor,  at  some  distance,  would  observe 
its  curvilinear  direction. 

The  power  of  gravitation  is  greatest  at  the  surface 
of  the  earth,  and  decreases  both  upwards  and  down- 
wards, but  in  a  different  proportion.  In  ascending^ 
the  gravity  decreases  as  the  squares  of  the  distance 
from  the  centre  increase ;  for  at  the  distance  of  the 
earth's  semidiameter  from  its  surface,  the  gravity  is 
not  more  than  a  fourth  of  that  power  on  its  surface. 
The  force  of  gravity  downwards  from  the  earth^s 
surface  is  in  a  direct  ratio  as  the  distance  from  the 
centre;  for  at  half  the  semidiameter  from  its  centre, 
the  power  of  gravity  is  only  equal  to  half  the  power 
at  the  surface ;  at  a  quarter,  one  fourth ;  and  so  on  for 
any  given  distance  in  like  proportion. 

The  power  of  gravity  retards  bodies  that  are  thrown 
upwards  in  the  same  proportion  that  it  accelerateii 


Collision^  ^c. 


31 


those  that  fall ;  so  that  the  times  of  ascent  and  descent 
are  equal  to  and  from  the  same  height. 

For  if  a  stone  be  thrown  from  d  to- 
wards B,  and  B  c  be  the  perpendicular 
line  of  gravitation ;  the  stone  will  be  re- 
tarded in  its  ascent,  or  the  gravity  b  c 
will  overcome  the  impetus,  in  propor- 
tion to  the  decreasing  parallels,  a  10; 
bG;  r  8,  &c.  till  the  whole  projecting 
force  is  destroyed.  When  the  stone  re- 
turns it  will  fall  from  b  towards  c,  with 
an  accelerating  force,  increasing  as  the 
parallels  increase,  till  it  reaches  the  sur- 
face at  D :  so  that,  according  to  the  laws 
of  gravity,  the  ascending  and  descend- 
ing times  are  equaL 


Collision  of  Elastic  a?id  JVonelastic  Bodies. 

The  collision  of  these  bodies  chiefly  depends  on 
the  third  law  of  motion,  in  which  action  and  reaction 
are  equal  and  contrary.  For  if  two  bodies  strike  each 
other,  their  motions  are  equally  affected,  and  their 
collision  produces  equal  changes,  but  in  contrary 
directions. 

Bodies  which  are  devoid  of  elasticity  will  not  sepa- 
rate after  the  stroke,  but  will  move  on  with  half  the 
motion^  that  the  striking  body  had  acquired  before 
the  impact ;  supposing  the  bodies  to  be  equal. 

*  By  the  term  motion,  in  a  physical  sense,  is  most  frequently  un- 
derstood, the  product  of  velocity  and  weight,  in  which  sense  the 
motion  of  the  two  bodies  after  the  stroke,  will  be  the  same  as  that 
of  the  striking  body  before  the  stroke;  though  they  will  move 
with  but  half  liie  velocity.  Ed. 


52      Collision  of  Elastic  and  Nonelastic  Bodies. 


Let  two  balls  a  b,  made 
of  clay  or  any  other  non- 
elastic  body,  be  suspended 
in  such  a  manner  that  their 
surfaces  may  just  touch 
each  other  when  they  are 
at  rest :  then  let  a  fall  from 
any  given  height,  and  it  will  acquire  such  a  velocity 
in  describing  the  arc  a  b,  as  would  carry  it  to  an  equal 
height  c,  if  it  were  not  obstructed  by  the  other  ball 
b:  but  on  striking  b,  which  is  of  equal  magnitude,  it 
communicates  one  half  of  its  motion,  and  the  two  balls 
move  on  together  to  Ezii  b  c.  In  experiments  of  this 
kind,  whether  with  elastic  or  nonelastic  bodies,  the 
theory  will  vary  in  some  degree  from  the  practice, 
not  only  from  the  imperfection  of  the  bodies,  but  from 
the  resistance  of  the  medium  through  which  they  fall. 

What  is  meant  by  the  collision  of  elastic  bodies  is, 
when  the  particles  give  way  at  the  point  of  impact, 
but  restore  themselves  to  their  first  position  after  the 
pressure  is  removed.  The  most  perfect  elastic  bodies 
are  ivory,  glass,  hardened  steel,  and  some  compound 
metals. 

To  show  the  elasticity  of  these  bodies :  If  two  balls 
of  the  above  description  be  suspended  from  a  com- 
mon centre,  and  one  of  them  have  its  surface  thinly 
painted ;  then  as  it  hangs  in  contact  with  the  other,  it 
will  make  a  small  mark  on  the  side;  but  if  the  painted 
ball  be  raised  to  a  certain  height,  and  then  suffered 
to  fall  against  the  other,  the  second  impression  on  the 
receiving  ball  will  be  considerably  larger  than  the 
first,  which  could  not  have  happened  if  the  elasticity 
of  the  balls  had  not  suffered  them  to  be  indented  by 
collision. 

When  a  body,  which  is  perfectly  elastic,  strikes 
another  of  the  same  kind  and  magnitude ;  the  striking 
body  will  communicate  the  whole  of  its  motion  to  the 
other,  and  afterwards  remain  at  rest. 


Collision  of  Elastic  and  Nonelastic  Bodies.      33 

Let  two  balls  of  ivory,  c  b,  be  suspended  from  a  ; 
on  suffering  b  to  fall  freely 


on 


c,  it  will  lose  the  whole 


of  the  motion  which  it  had 

acquired    in    its    descent 

through  the  arc  b   c,  and 

c  will  be  driven  up  to  d 

ill  the  same  manner  as  b 

would  have  been  carried 

to  that  point  by  its  acquired  velocity,  if  it  had  not  been 

obstructed  by  c. 

If  four,  or  any  other  number  of  equal  elastic  balls, 
be   suspended   on  centres  near 
each  other,  and  d  be  let  fall  from 
any   given   height;    then  as   it 
strikes  c,  it  will  communicate 
the  whole  of  its  motion  to  it;  and 
this  will  pass  through  the  cen- 
tres of  c  and  b  to  a  ;  leaving  b, 
c,  D  quiescent;  whilst  a  is  dri- 
ven off  with  a  velocity  equal  to 
have  been  communicated  by  d, 
obstructed  by  the  intervening  bodies  b  c. 

Again,  if  eight  such  balls  were  suspended,  and  two 
were  suffered  to  fall  at  the  same  time,  but  at  a  small 
distance  from  each  other;  the  middle  balls,  from  the 
equal  power  of  action  and 


that  which  would 
if  it  had  not  been 


'/ 


reaction,  would  remain 
at  rest,  and  the  motion 
would  pass  through  their 
centres  to  the  two  remote 
balls,  which  will  indivi- 
dually possess  the  same 
motion  as  if  no  obstruc- 
tion had  intervened. 
Whatever  number  of  balls  may  be  let  fall  on  one 
side,  the  same  number  w^ill  have  an  equal  motion  on 
the  other,  and  the  intermediate  balls  will  remain  mo- 
tionless. 


iycr-- 


\Pendulums. 

A  PENDULUM  is  a  heavy  body  suspended  by  a 
small  cord,  or  a  piece  of  wire,  which  moves  about  a 
point  as  its  centre.  When  this  weight  is  put  in  mo.- 
tion,  it  will  descend  through  one  half  of  an  arc  by  its 
own  gravity,  and  ascend  the  other  half  by  the  veloci- 
ty which  it  has  acquired  in  its  descent. 

If  A  be  a  weight  suspended  by  a  line,  or  wire, 
from  the  centre  or  point  ^j 

of    suspension   c ;   when 
it  is   let  fall   from    d,   it  y^ 

will  descend  through  the  y^ 

arc  D  A,  by  its  own  gra-     ^" ^• 

vity,   and    acquire    such         " .. 

a    velocity   as    it    would .;. :::.-. -.....(4). 

obtain  in  falling  from  e  to     '  ^^- 

A.  By  the  first  law  of  motion  the  weight  would  fly  off* 
in  a  tangent,  or  straight  line  a  f,  if  it  were  not  re- 
tained by  the  string;  which,  with  the  velocity  the 
weight  has  acquired  at  .a.,  conducts  it  to  b,  and  the 
whole  arc  d  b,  forms  one  oscillation.  At  b,  which  is 
in  the  same  horizontal  line  with  the  opposite  point  d, 
it  loses  its  motion,  and  then  returns  back  again  by 
the  force  of  its  gravity:  thus  the  pendulum  would 
continue  moving  for  ever,  if  it  were  not  perpetually 
retarded  by  the  friction  of  the  cord  upon  the  point  of 
suspension,  and  the  resisting  medium  of  the  air, 
through  which  it  vibrates;  but  these  gradually  retard 
its  motion,  so  that  each  oscillation  becomes  some- 
thing less  than  the  preceding  one,  till  at  length  the 
ball  rests  in  the  line  of  suspension  c  a. 

The  time  of  the  vibration  of  pendulums  is  as  the 
square  root  of  their  lengths;  that  is,  if  one  pendulum 
be  four  times  longer  than  another,  it  will  vibrate  half 
as  fast;  if  it  be  nine  times  longer,  one  third  as  fast; 
if  sixteen  times,  one  fourth  as  fast;  and  so  on  accord- 
ing to  the  square  root  of  the  length,  as  before  stated. 


Pendulums,  ^^  35 

The  centrifugal  force,  or  rotatory  motion  of  the 
earth  round  its  axis,  which  causes  it  to  swell  out  at 
the  equator,  and  flattens  it  towards  its  poles,  likewise 
causes  pendulums  to  vibrate  in  unequal  times,  in 
different  latitudes.  For  as  the  repulsive  or  centrifugal 
force  is  greatest  at  the  equator,  and  lessens  gradually 
towards  the  poles,  the  vibration  of  the  same  pendu- 
lum will  become  slower  by  the  increasing  resistance, 
as  it  is  taken  tow^ards  the  equator,  and  faster  by  the 
decreasing  power,  as  it  approaches  the  poles  of  the 
earth.* 

A  pendulum  of  39.2  inches  in  length,  vibrates  se- 
conds in  the  latitude  of  London ;  and  as  a  second  forms 
an  aliquot  part  of  the  time  occupied  in  a  diurnal  re- 
volution of  the  earth,  the  utility  of  the  pendulum  is 
sufficiently  obvious,  in  marking  the  different  divisions 
of  that  period  of  time. 

All  metalline  bodies  expand  and  contract  by  heat 
and  cold,  which  prevents  the  common  wire  pendulum 
from  being  always  quite  correct  in  its  length,  even  in 
the  same  latitude ;  this  is  partly  remedied  by  forming 
the  rod  of  bars  of  different  metals,  the  various  actions 
of  which  counteract  each  other,  and  render  it  more 
permanently  accurate  in  its  dimensions.  A  uniform- 
rod,  one-thirdf  longer  than  a  pendulum  which  is  for- 
med by  a  wire  and  weight,  will  vibrate  in  the  same 
time,  and  the  centre  of  oscillation  in  the  weight  will 
be  the  centre  of  percussion  in  the  rod.  By  the  centre 
of  percussion,  is  meant  that  part  of  a  rod  or  stick 
which  produces  the  greatest  impression  in  striking  a 
blow. 

*  Both  on  account  of  the  spheroidal  figure  of  the  earth,  and  its 
diurnal  rotation,  gravity,  and  consequently  the  vibration  of  pendu- 
lums, will  gradually  increase  from  the  equator  to  the  poles ;  and 
therefore  pendulums,  to  keep  time,  must  be  made  of  different 
lengths,  according  to  the  latitude.  Ed. 

t  This  is  strictly  true  only  when  the  pendulum-rod  is  consi- 
dered as  a  mere  line ;  though  when  the  thickness  is  inconsiderable, 
•the  error  will  be  insensible .  Ed. 


56 


Centre  of  Gravity, 

The  centre  of  gravity  is  that  point  about  which  all 
the  parts  of  a  body  exactly  balance  one  another; 
therefore,  if  this  point  be  supported  the  body  will  be 
at  rest,  whatever  be  its  form ;  and  as  this  point  may 
be  conceived  as  the  concentration  of  the  weight  of  the 
body,  it  is  hence  called  the  centre  of  gravity. 

If  two  bodies  of  equal  w  eight  be  fastened  to  the 

extremities    of   a    ^ ^ 

uniform   rod,   the    w                   jl  w 
point    of   suspen- 
sion, or  centre  of  ^^          F            /^^ 

gravity,  will  be  in  ^^  J,  ^0 

the  middle,  or  equi- 
distant from  the  two  extremities  in  the  manner  of  a 
scalebeam.  But  if  the  bodies  be  of  unequal  weights, 
the  point  of  gravitation  in  the  rod  will  be  according 
to  these  weights;  that  is,  if  e  be  to  d,  as  3  to  1, 
then  F  E  will  be  to  n  f  as  1  to  3,  or  the  length  of 
the  arms,  from  the  point  of  suspension,  will  be  in- 
versely proportional  to  the  weights. 

The  centre  of  gravity  in  any  plane  figure  may  be 
found  in  the  following  man- 
ner. Let  the  body  be  freely 
suspended  by  a  string,  and  ap- 
ply a  plumb-line  to  A,  the  point 
of  suspension;  this  will  pass 
over  the  centre  of  gravity 
somewhere  in  the  line  a  b, 
which  falls  beneath  the  centre 
of  suspension;  then  mark  the 
line  A  B,  and  hold  the  plumb- 
line  again  from  c,  another 
point  of  suspension,  and  the 
point  E,  where  the  lines  cross  each  other,  will  be  the 
centre  of  gravity. 


Centre  of  Gravity, 


37 


iil|i|l!'liiitJlililliH!.'l'JliiliiliaW:t 


The  centre  of  gravity  in  a  square  or  flattened  bar 
is   easily   found   by 
balancing  it  on  the  / 

blunt  edge  of  a  knife. 
After  fixing  the 
knife,  balance  the 
bar  with  the  ends  a 

little  inclining  towards  it,  and  then  mark  the  line  of 
gravitation;  afterwards  reverse  the  position  and  ba- 
lance it  again,  and  the  point  where  the  edge  crosses 
the  first  line  is  called  the  point  of  suspension,  or  cen- 
tre of  gravity. 

If  a  body  be  placed  on  a  horizontal  plane,  and  a 
perpendicular  line  from  the  centre  of  gravity  fall  with- 
in its  base,  it  will  stand;  if  the  line  fall  beyond  the 
base,  the  body  cannot  support  itself.  For  if  a  bar  be 
placed  endwise  on  the  edge  of  a  table, 
and  the  line  of  direction  fall  within 
the  base  d  c,  the  centre  of  gravity 
will  be  supported  by  the  table,  and 
the  body  will  not  fall,  although  the 
top  overhang  its  base ;  but  if  it  be  still 
more  inclined,  so  that  the  line  of  di- 
rection A  c  comes  beyond  the  point 
c  of  the  base;  the  centre  of  gravity 
loses  its  support,  and  the  body  falls 
to  the  ground. 

Inclining  walls,  and  old  buildings, 
support  themselves  from  the  above 
principle,  and  remain  with  their  tops  overhanging 
their  bases  for  a  number  of  years. 


D 


38 


Centre  of  Gravity, 


The  tower  of  Pisa,  in  Italy, 
amongst  many  others,  is  a  remarka- 
ble instance,  in  proof  of  this  law  of 
gravitation ;  for  the  top  of  the  tower 
overhangs  its  base  sixteen  feet;  so 
that  strangers  pass  by  it  with  terror, 
lest  it  should  fall  on  their  heads ;  but 
as  the  line  of  direction  falls  within  the 
base  of  the  building,  it  still  remains 
supported;  and  as  it  has  stood  some 
centuries  in  this  state,  it  is  probable 
that  it  may  stand  many  more,  if  the 
cement  by  which  it  is  held  together  should  not  perish. 

A  body  stands  the  most  firmly  when  the  base  is 
broad  and  the  line  of  direction  falls  in  its  centre ;  con- 
sequently the  more  narrow  the  base,  and  the  more  the 
line  of  direction  approaches  its  extremity,  the  greater 
is  its  danger  of  failing. 

If  a  piece  of  board  be  placed  on  the  edge  of  a  table, 
and  the  line  of  gravity  fall  be-  • 
yond  it,  it  will,  of  course,  drop 
to  the  ground;  but  if  two 
wires,  loaded  at  one  end,  have 
the  other  fastened  to  the  up- 
per side  of  the  board,  and  the 
weights  rest  over  the  table; 
the  line  of  direction  will  fall 
nearer  the  weights,  and  the  board  will  remain  suppor- 
ted by  the  edge  of  the  table. 

If  A  be  a  double  cone,  and  the  radius  of  its  base,  c 
A,  be;  greater  than  the 
height,  B  D,  of  the  inclined 
plane,  b  c,  and  c  e:  on 
placing  it,  towards  the  an- 
gular point,  between  the 
sides  of  the  plane,  the  cone 
will  ascend,  and  the  centre 
of  gravity  seem  to  move  upwards,  but  the  reverse  is 


The  Lever.  39 

the  fact ;  for  the  centre  of  gravity  descends  with  the 
cone  as  it  rolls  towards  its  extremities,  between  the 
diverging  sides  of  the  wire.  If  the  height  of  the  plane 
B  D  be  equal  to  the  radius,  or  half  the  thickest  part  of 
the  double  cone,  the  cone  will  remain  at  rest  on  any 
part  of  the  frame.  If  the  height  be  made  greater  than 
the  radius,  it  will  descend  towards  the  angular  point, 

A 

Mechanic  Powers. 

These  powers  are  simple  instruments  or  machines 
in  the  hands  of  man,  by  which  he  is  enabled  to  raise 
great  weights,  and  overcome  such  resistances,  as  his 
natural  strength  could  never  effect  without  them. 

The  mechanic  bodies  are  generally  included  in  the 
Lever,  Wheel  and  Axle^  Pulley,  Inclined  Plane, 
Wedge,  and  Screw, 

Compound  machines  are  formed  from  these  simple 
powers;  even  the  most  complex  machine  is,  in  a  me- 
chanical sense,  nothing  more  than  a  combination  of 
the  above  simple  forces. 

In  considering  their  operations  theoretically,  the 
principles  are  mathematically  just:  but  in  a  practical 
application,  some  allowances  must  be  made  for  weight, 
thickness,  and  friction. 

The  Lever, 

Th  IS  is  the  most  simple  of  all  the  mechanic  powers ; 
and  is  called  a  handspike  when  it  is  made  of  woold,  or 
a  crow  when  it  is  formed  of  iron.  The  operation  of 
this  engine  is  considered  in  three  different  ways.  The 
first  of  these  is,  when  the  weight  which  is  to  be  mov- 
ed, lies  on  one  side  of  the  fulcrum,  or  prop,  and  the 
power  which  is  given  by  the  arm,  weight,  &:c.  lies  on 
the  other. 

The  operation  of  a  lever  of  this  description  may  be 
represented  by  a  common  poker,  in  the  act  of  stirring 


40  The  Lever. 

the  fire.  Here,  the  poker  is  the  lever;  the  bar  of  the 
grate,  the  bearing  place,  or  j)rop ;  and  the  fuel  contain- 
ed in  it,  the  weight  to  be  raised. 

The  second  description  is,  when  the  weight  is  be- 
tween the  prop  and  the  power.  When  a  large  stone, 
or  an\  other  heavy  body,  is  forced  forwards  with  an 
iron  crow,  the  point  of  the  instrument  is  stuck  in  the 
ground;  this  is  considered  as  the  prop;  the  other  ex- 
tremity is  acted  upon  by  the  power,  and  the  resis- 
tance, or  weight,  comes  against  the  instrument,  at 
some  distance  from  its  point,  that  is,  between  the  prop 
and  the  power. 

The  third  kind,  is  when  the  power  is  applied  be- 
tween the  prop  and  the  weight:  thus,  if  a  ladder  be 
raised  with  one  end  on  the  ground;  the  lower  end  is 
taken  as  the  prop;  the  other  extremity  is  taken  as  the 
weight  to  be  risen;  and  the  force  of  the  hands  which 
pull  at  the  bars,  is  the  power  which  falls  .between  the 
prop  and  the  weight. 

The  lever  a  b,  is  of  the  first  kind,  and  has  the  ful- 
crum, or  prop,  c,  be-   ^  ^^^ 

IB 


^      <3      4r      5     6 


tween  the  weight  and 
the  power:  the  short- 
er arm,  a  c,  is  made 
thick  and  heavy,  that 
it  may  balance  the 
longer  end  c  b,  so  that  the  lever  may  be  considered 
without  weight,  as  it  turns  upon  the  prop  c.  The  ad- 
vantage or  additional  power  acquired  by  the  use  of 
the  lever  is  in  proportion  to  the  difference  of  the  arms ; 
for  if  the  arm  c  b,  be  divided  into  six  parts,  and  the 
other  arm,  a  c,  be  equal  to  one  of  these  parts;  a 
Veight  of  six  poinds  suspended  from  a,  will  be  ba- 
lanced by  hanging  one  pound  at  b,  the  opposite  end. 
Here  the  six  pounds  may  be  taken  as  the  resistance, 
and  the  opposite  pound  as  the  power;  so  that  the  re- 
sistance is  as  much  nearer  to  the  prop,  as  the  weight 
or  power  is  lighter  than  the  resistance.  In  every  case 


The  Lever.  41 

of  the  lever,  if  the  product  of  the  resistance  and  lever^ 
at  one  end,  be  equal  to  the  product  of  the  lever  and 
power  at  the  other,  the  lever  will  rest  in  equilibrium. 

Suppose  a  stone  of  a  ton  weight,  to  be  fastened  by 
a  rope  to  the  end  of  a  lever  10|  feet  long,  and  the  prop 
to  be  placed  under  it,  at  the  distance  of  six  inches 
from  the  end;  the  power  of  a  hundred  weight  at  the 
other  extremity  will  raise  the  weight  from  the  ground. 
Thus,  by  increasing  the  longer  arm,  even  the  ordina- 
ry power  of  a  man  may  be  made  equal  to  the  greatest 
resistances. 

A  balanced  scale-beam  is  a  lever  of  the  first  kind, 
with  its  prop  placed  in  the  middle,  and  the  centre  of 
gravity  directly  under  it.* 

If  two  bodies,  f  and  g,  of  equal  weight,  be  suspen- 
ded at  equal  distances  from  the  prop,  the  centre  of 
gravity  will  still  rest 
beneath  the  point  of 
suspension;  for  as  the 
weights  and  distances 
are  equal,  the  powers  are 
equal;  therefore,  nei- 
ther end  can  preponde- 
rate. But  if  F  be  moved 
to  K,  the  centre  of  gravity,  e  moves  towards  c,  and 
falls  beyond  d,  the  point  of  suspension;  in  which  case 
it  becomes  unsupported,  and  the  end  of  the  beam  c, 
descends  towards  the  line  d  e  ;  for  by  an  invariable 
law  of  nature,  the  centre  of  gravity  is  impelled  to- 
wards the  line  of  suspension.  This  likewise  shows 
that  a  smaller  weight  than  g  will  balance  the  opposite 

*  In  the  construction  of  scale-beams,  great  care  must  be  taken 
to  place  the  centre  of  motion  and  the  two  centres  of  suspension, 
(at  the  extremities  of  the  beam)  in  the  same  right  line;  otherwise, 
when  the  beam  is  moved  out  of  its  horizontal  position,  one  end 
will  approach  nearer  to  the  vertical  line  passing  through  the  cen- 
tre of  motion  than  the  other,  and  the  lesser  weight  might  actually 
appear  to  preponderate.  Ed. 


K        P 


42  The  Lever, 

weight  F,  when  it  is  suspended  from  k.  The  steelyard 
acts  on  this  principle,  for  if  the  longer  end,  d  c,  were 
equal  to  six  times  the  shorter  k  d  ;  then  a  body  sus- 
pended from  c,  would  balance  six  times  its  weight, 
suspended  from  k. 

Scissars,  snuffers,  pincers,  &c.  are  made  up  of  two 
levers,  and  the  prop  is  the  rivet  that  fastens  them  to- 
gether. 

The  second  kind  of  lever  has  the  resistance  and 
power  on  the  same  side  of  the  prop.  The  advantage 
gained  by  levers  of  this  kind,  is  still  in  proportion  to 
the  difference  between  the  shorter  and  longer  division. 

For  if  one  end  of  the  lever,  a  b  c,  be  supported  at 
A,  and  A  c  be  seven  times  as  long  as  a  b;  seven 
pounds  suspen- 
ded at  D,  will 
be  balanced  by 
one  pound  at  g 


the  opposite  end:  ',„    A  n^m 

thus,  the  com- 
mon power  of  a 
man  would  be  increased  seven  fold  in  raising  any 
great  weight  suspended  from  b;  but  to  raise  the 
weight  a  foot,  or  an  inch,  from  b  to  e,  the  end  c  must 
pass  through  seven  times  that  space,  or  from  c  to  f  ; 
so  that,  what  is  gained  in  power  is  lost  in  time,  in  this, 
as  well  as  in  every  other  augmentation  of  force  by 
mechanic  power. 

A  wheei-barrow  may  be  considered  as  a  lever  of 
the  second  order;  a  being  the  place  of  the  wheel,  c 
the  handles,  and  b  the  load  in  the  barrow ;  under  this 
augmentation  of  power,  a  much  greater  weight  may 
be  moved,  by  means  of  this  machine,  than  could  be 
supported  on  the  shoulders  of  a  single  person. 

A  patten- maker's  knife  acts  as  a  lever  of  the  same 
kind;  for  the  end  which  is  fastened,  is  the  prop,  the 
power  is  at  the  handle,  and  the  wood  which  is  cut, 
the  resistance.  Here  the  power  is  increased  as  the  re- 
sistance approaches  the  prop. 


Wheel  and  Axis.  43 

The  third  order  of  levers  has  its  power  between  the 
prop  and  the  weight,  and  may  be  considered  as  the 
reverse  of  the  second ;  for  the  power  which  is  applied 
must  exceed  the  weight,  as  much  as  the  distance  of 
the  weight  from  the  prop  exceeds  the  distance  of  the 
power  from  the  same  point. 

If  the  lever  a  b  be  divided  into  seven  equal  parts; 
with  one  end  a,  fixed  under  a  table,  and  a  ball  of  one 
pound  be  sus- 
pended from  the 
opposite  end,  it 
will  require  a 
force  of  seven 
pounds,  pulling 
upwards  from 
the  first  division 

c,  to  balance  the  opposite  power.  Here  the  smaller 
weight,  in  rising  or  falling,  has  seven  times  the  velo- 
city, and  describes  seven  times  the  space,  of  the 
larger. 

No  mechanical  advantage  is  gained  by  this  kind  of 
lever;  for  as  the  power  must  always  exceed  the  weight, 
it  is  rarely  used:  a  pair  of  wool-shares,  which  act  by 
a  pressure  in  the  middle,  is  a  lever  of  this  description. 
But  this  order  seems  greatly  employed  in  the  action 
of  the  animal  body ;  for  the  bones  are  levers,  the  joints 
pf  them  the  fulcra,  and  the  muscles  the  power  that 
gives  motion  to  the  whole. 

Wheel  and  Axis. 

This  machine  may  be  considered  as  a  kind  of  per- 
petual lever,  having  its  fulcrum  or  prop  in  the  centre 
of  the  axis  and  the  wheel.  The  acting  or  longer  part 
of  the  lever  is  the  radius  or  half  the  diameter  of  the 
wheel,  and  the  shorter  or  resisting  part,  is  the  radius 
of  the  axis ;  the  power  or  weight  falls  perpendicularly 
from  the  circumference  of  that  part  of  the  wheel  which 
is  in  a  horizontal  line  with  the  axis. 


44 


Wheel  and  Axis. 


In  the  figure  let  a  be  the  axis,  the  radius  of  which 
is  6  inches,  and  a  b,  48  inches, 
the  semidiameter  of  the  wheel: 
then  a  weight  of  eight  pounds 
suspended  from  a,  will  be  coun- 
terbalanced by  one  pound  hanging 
from  B ;  for  as  the  acting  part  of 
the  lever,  or  radius  of  the  w^heel, 
is  eight  times  as  long  as  the  resist- 
ing part,  or  radius  of  the  axle,  # 
the  power  c  will  balance  eight  ^ 
times  its  weight,  suspended  from 
A,  which  accords  with  the  principle  of  the  lever.  The 
advantage  gained  by  the  power  is  lost  in  the  time,  as 
in  the  preceding  examples;  for  whilst  d  ascends 
through  the  space  of  a  foot,  c  must  descend  through 
eight  times  that  distance. 

There  are  many  modes  of  applying  this  mechanical 
power;  as,  by  a  handle  which  is  used  to  wind  up  a 
jack,  or  raise  a  bucket  of  water  in  a  well;  here  the 
advantage  is  in  proportion  to  the  thinness  of  the  roller 
and  length  of  the  crank,  or  the  distance  of  the  hand 
or  power  from  the  axis.  The  capstan  is  another  ma- 
chine formed  on  this  principle ;  the  upright  post  which 
turns  on  a  spindle,  and  receives  the  coil  of  the  rope, 
may  be  considered  as  the  axis  of  the  wheel,  and  the 
levers  which  are  fixed  into  the  post  the  radii,  at  the 
extremities  of  which  the  power  is  applied.  The  pow- 
er which  is  gained  by  this  machine  increases  as  the 
diiference  increases  between  the  radius  of  the  upright 
post  and  the  length  of  the  levers  which  turn  it.  xVs 
the  coils  of  rope  increase  upon  the  post,  the  radius 
of  the  axle  becomes  larger,  and  therefore  lessens  the 
force  of  the  lever.  Cranes  of  various  forms,  for  load- 
ing and  unloading  goods,  as  well  as  capstans,  arc 
generally  constructed  according  to  the  principle  of 
the  axis  and  wheel. 


45 


The  Pulley, 

This  mechanical  power  is  formed  by  a  small  wheel, 
made  of  wood  or  metal,  with  a  groove  in  its  circumfer- 
ence, which  is  placed  in  a  frame  and  turns  on  an  axis. 

The  wheel  is  called  the  sheave ;  the  axis  on  which 
it  turns,  is  the  gudgeon  or  pin;  and  the  frame  in 
which  it  is  placed  is  called  the  block. 

Let  the  pulley  a  support  two  equal  weights,  p  and 
w,  from  the  ends  of  a  cord 
which  passes  in  a  groove  over 
the  sheave ;  then  p  w  will  coun- 
terpoise each  other,  and  the  bo- 
dies will  be  at  rest,  in  the  same 
manner  as  if  the  cord  was  cut 
into  two  parts,  and  each  tied  to 
the  end  of  a  balanced  beam; 
which  is  represented  by  the  dot- 
ted line  DAE. 

Therefore,  supposing  p  to  be  the  power  and  w  the 
weight,  a  single  pulley  does  not  increase  or  diminish 
the  mechanic  eifect. 

The  principal  advantage  which  arises  from  a  single 
pulley,  is  in  the  mode  by  which  the  power  is  applied; 
for  in  raising  a  loaded  basket  or  bucket,  by  a  line 
passed  over  a  pulley,  the  force  acts  downwards ;  which 
is  much  less  laborious  than  when  it  is  applied  in  an 
opposite  direction. 


46 


The  Pulley. 


When  one  end  of  the 
tackle  or  cord  is  fastened 
to  a  hook  in  the  beam, 
and  the  other  passes 
through  the  groove  of 
the  moving  sheave  h, 
and  from  this  over  ano- 
ther, fixed  at  i;  the  power 
K  will  support  double  its 
own  weight  suspended  at 
L ;  for  it  is  obvious  that 
the  cords  m  and  n  each 
support  one  half  of  the 


/Sk 


weight;  therefore,  half  the  weight  of  l,  which  is  sup- 
ported by  N,  is  again  counterpoised  by  k:  this  being 
equal  to  half  the  weight  supported  by  n,  it  is  likewise 
equal  to  half  the  weight  of  l,  and  the  power  gained  is 
in  the  proportion  of  two  to  one.  Here  again  the  ad- 
vantage which  is  gained  by  the  powxr  is  lost  in  the 
time;  as  the  power  k  moves  through  twice  the  space 
of  the  weight  l.  For  if  the  pulley  h  rise  a  foot,  the 
cords  M  and  n  must  be  shortened  a  foot  each,  which 
gives  two  feet  to  k,  the  descending  power.  This  dif- 
ference of  motion  increases  according  to  the  number 
of  sheaves  in  the  block;  for  if  there  were  three 
sheaves  in  each  block,  there  would  be  six  cords,  and 
the  power  would  descend  six  times  as  fast  as  the, 
weight  would  rise. 


The  Pulley. 


47 


If  a  fixed  and  a  moving  block  have  each  of  them 
three  sheaves,  and  a  weight  be  sus- 
pended from  the  lower  block,  it  will 
be  equipoised  by  a  power  equal  to 
one  sixth  of  the  weight ;  for  as  the 
weight  is  supported  by  six  lines  from 
the  three  sheaves  in  each  block,  the 
weight  is  equally  divided  amongst 
them,  in  the  same  manner  as  in  the 
preceding  example.  So  that  the  pow- 
er obtained  is  as  six  to  one,  or  a 
weight  of  a  hundred  pounds  at  p,  will 
balance  six  hundred  at  w ;  but  p  must 
move  through  six  times  the  space  of  w. 

When  the  sheaves  are  not  fastened  together  in  the 
Ipwer  block,  but  act  upon  each  other,  and  the  weight 
is  fastened  to  the  lowest,  the  power  will  be  greatly 
increased. 

If  one  end  of  the  cords  which  pass  through  the 
four  pullies,  be  fastened  to  a 
beam,  and  the  other  to  the  block 
of  the  adjoining  pulley,  the 
weight  would  be  divided  in  such 
a  manner  amongst  the  different 
pullies,  that  sixteen  pounds  at 
w,  would  be  balanced  by  two 
pounds  at  p.  For  if  the  cord  g  c 
suspend  sixteen  pounds  from 
the  block  w,  according  to  what 
has  been  said,  each  part  of  the 
line  supports  one  half  of  the 
w^eight,  and  the  half  which  is 
supported  by  c  is  again  divided 
into  two  equal  parts  by  y and  ^, 
and  b  sustains  one  half  of  c^  or 
a  fourth  of  the  whole  weight  w;  the  weight  at  ^,  is 
again  divided  and  balanced  by  p  the  power :  so  that 
the  power  is  equal  to  the  whole  weight  at  w ;  a  quar- 
ter at  c,  and  one  eighth  at  b.  The  uppermost  pulley 


48  The  Wedge. 

gives  no  increase  of  power,  and  it  is  placed  merely 
for  the  convenience  of  pulling  downwards  frbm  p, 
rather  than  upwards  at  a. 

There  are  many  other  combinations  of  blocks  to 
diminish  the  weight  and  give  increase  to  the  power; 
but  a  number  of  sheaves  in  the  same  block,  not  only 
lose  much  in  the  time,  but  are  obstructed  in  their 
operation  by  the  cheeks  of  the  block,  from  applying 
the  power  to  the  outside  sheave.  This  inconvenience, 
which  arises  from  having  a  number  of  sheaves  in  the 
same  block,  has  been  greatly  lessened  by  an  invention 
of  Smeaton,  who  makes  the  rope  that  sustains  the 
power,  proceed  from  the  middle  sheave  in  the  block, 
by  which  means  the  power  acts  upon  all  the  sheaves 
in  .the  same  parallel  direction. 

The  Wedge. 

The  wedge  is  usually  made  of  iron  or  wood,  and 
is  used  for  raising  great  weights,  or  separating  firm 
blocks  of  wood  or  stone,  by  driving  it  into  the  mass 
with  a  hammer. 

The  doctrine  of  the  wedge  has  been  differently 
considered,  by  different  authors ;  but  as  there  are  so 
many  natural  obstructions  to  overcome  in  its  opera- 
tions; such  as  elasticity,  tenacity,  and  friction;  the 
practical  effect  is  inadequate  to  serve  as  a  proof  of  the 
theory.  It  is,  however,  generally  considered  that  the 
power  which  acts  against  the  back,  is  to  the  force 
acting  perpendicularly  against  each  side,  as  the  breadth 
of  the  back  of  the  wedge  is  to  the  length  of  its  side. 


Liclined  Plane, 


45 


Let  A  and  b,  two  cylinders,  be  suspended  from  €, 
and  let  lines  from 
the  axis  of  each 
cylinder  pass  over 
the  pullies  d  e; 
those  from  the  cy- 
linder B  passing 
over  D  and  from  a 
over  E,  each  hav- 
ing a  weight  of 
two  pounds  fasten- 
ed at  F  and  g  ;  then  the  cylinders  will  be  drawn  to- 
gether and  form  a  resistance  to  the  sides  of  the  wedge 
A  K  and  K  B :  Now,  supposing  the  resistance  of  the 
cylinders  to  act  with  a  force  of  two  pounds  each,  and 
the  length  of  the  wedge  b  k  to  be  equal  to  twice  its 
thickness  a  b  ;  the  pressure  of  two  pounds  on  the  top 
of  the  wedge  i  will  be  equivalent  to  four  pounds,  the 
resistance  from  the  sides  of  the  cylinders. 

But  the  force  that  is  given  to  a  wedge  is  generally 
applied  by  a  stroke,  and  not  by  the  dead  pressure  of 
a  weight ;  and  a  smart  blow  with  a  hammer  of  four 
ounces  will  overcome  more  resistance  than  a  weight 
of  two  pounds,  laid  on  the  top  of  the  wedge.  A  blow 
from  a  hammer  of  fourteen  pounds  weight  will  over- 
come more  resistance  in  cleaving  a  log  of  wood,  than 
the  pressure  of  a  ton  weight  laid  on  the  top  of  the 
wedge :  the  suddenness  of  the  blow  causes  a  more 
easy  separation  of  the  cohesive  parts,  and  the  power 
of  the  hammer  is  greatly  increased  by  the  multiplica- 
tion of  its  velocity  into  its  weight. 


Inclined  Plane, 

This  may  be  considered  as  a  stationary  wedge,  as 
it  possesses  the  same  properties ;  for  the  power  which 
is  gained  by  the  inclined  plane  is  as  the  length  of  the 
plane  to  its  height. 


50  Inclined  Plane, 

If  the  perpendicular  height  b  c  be  equal  to  one 
third  of  the  base 
A  c,andif  thehne 
which  passes  over 
the  pulley   e    be  y-^    ^. 

fastened     to    the  sC^'^  " 

axis  of  a  cylinder  ^g^^^;.,^„,, 

of   three   pounds  ^  5  ^  ..^...^^^^ 

weight  at  d  ;  one 
pound  suspended  at  the  opposite  end  of  the  cord  f 
will  equipoise  the  three  pounds  at  d,  and  the  two 
powers  will  be  at  rest. 

Thus  the  relative  gravity,  or  the  descending  power 
of  D,  is  in  the  same  proportion  to  its  actual  gri;vity, 
as  the  height  of  the  inclined  plane  c  b  is  to  the  length 
of  the  horizontal  plane  a  c. 

Now,  if  B  c  be  the  perpendicular  height  of  part  of 
a  hill,  and  equal  to 
one  half  of  d  c  and 
one  third  of  a  c  the  A 
horizontal  planes 
from  different  sides 
of  the  road ;  a  car- 
riage may  be  drawn 
up  with  one  third  less  power,  by  crossing  the  road 
from  A  to  B,  than  it  would  require  in  ascending  di- 
rectly from  D  to  B.  This  is  sufficient  to  show  the 
great  advantage  loaded  carriages  obtain  in  ascending 
hills,  by  crossing  from  one  side  of  the  road  to  the 
other:  but  here  it  must  likewise  be  observed,  as  in 
every  other  part  of  mechanics,  that  what  is  gained  by 
the  power  is  lost  in  the  time,  inasmuch  as  the  line  a 
B  is  longer  than  d  b. 


51 


Screw. 

This  machine  is  a  spiral  thread  or  groove  cut  round 
a  cylinder :  when  the  spiral  is  formed  upon  the  cylin- 
der, it  is  called  a  male  screw ;  but  when  it  is  cut  in 
the  inner  surface  of  a  hollow  cylinder  it  is  called  a 
female  screw.  If  the  spiral  thread  wxre  unfolded  it 
would  form  an  inclined  plane ;  the  length  of  which 
would  be  to  its  height  as  the  circumference  of  the  cy- 
linder is  to  the  distance  between  the  threads. 

For  if  the  circumference  d  b  be  five  inches,  and  it 
is  required  to  place  the  threads  at  the  distance  of  half 
an  inch  from  each  other;  it 
is  obvious  that  the  spiral, 
in  winding  round  the  cir- 
cumference, must  pass 
through  a  line  five  inches 
long  in  attaining  half  an 
inch  up  the  cylinder,  which 
is  the  distance  between  the 
threads.  Therefore,  if  the 
inclined  plain  g  f  h,  which 
is  half  an  inch  hi  height,  and  B^ 
five  inches  long,  be  cut  out  and  pasted  on  the  cylin- 
der, it  will  form  one  complete  revolution  of  the  spi- 
ral. Or  if  E  F  and  c  f  be  five  times  g  f  and  f  h,  the 
paper  will  roll  five  times  round  the  cylinder,  and  the 
edge  of  the  inclined  plane  will  form  a  spiral  or  thread, 
half  an  inch  wide  and  two  inches  and  a  half  in  extent. 

As  the  power  of  the  inclined  plane  is  as  its  length 
to  its  height,  so  the  power  of  the  screw,  is  as  the  cir- 
cumference of  the  cylinder  to  the  distance  of  the 
threads. 

To  compute  the  acting  force  of  the  screw.  If  the 
threads  on  the  cylinder  a  b  be  made  a  quarter  of  an 
inch  apart,  and  the  nut  d  be  turned  by  the  handle  or 
lever  c  d,  which  is  two  feet  long:  then,  as  the  power 


52 


Friction  or  Atti'ition. 


of  the  screw  is  relatively,  as 
the  circumference  is  to  the  dis- 
tance of  the  threads,  this  dia- 
meter of  the  power  will  be 
twice  c  D  or  48  inches,  and  its 
circumference  about  150  in- 
ches; so  that  the  power  is  to 
the  force  or  pressure  as  150  to 
|,  or  as  600  to  1.  Now,  let 
150/<^.  the  ordinary  force  of  a 
man,  be  applied  to  c,  the  end 
of  the  lever,  and  the  pressure 
on  the  block  b  will  be  600, 
multiplied  by  150  or  90000/^^. 


This  is  taken  independently  of  friction ;  but  as  the 
friction  of  the  screw  is  very  great  in  its  practical  ef- 
fect, the  pressure  will  fall  short  by  one  third  at  least 
of  the  above  calculation. 

It  has  been  stated,  and  with  seeming  propriety,  that 
although  the  operations  of  the  six  preceding  powers 
dift'er  from  each  other,  the  principles  are  all  reducible 
to  the  lever,  or  wedge ;  and  that  the  pulley,  wheel, 
and  axle,  belong  to  the  former,  and  the  inclined  plane 
and  screw  to  the  latter.  But  as  the  screw  has  its  in- 
fluence from  the  lever  as  well  as  the  power  of  the 
wedge,  may  it  not  be  considered  as  the  perfection  of 
mechanic  force  ? 


Frictio7i  or  Attrition. 

Friction  is  the  resistance  that  a  moving  body 
meets  with  from  the  surface  over  which  it  passes.  A 
carriage  wheel  as  it  turns  on  the  road  is  impeded  by 
the  inequalities  on  its  surface;  and  the  axle  on  which 
the  wheel  turns,  likewise  retards  it,  by  its  attrition  in 
the  nave.  The  friction  is  much  further  increased, 
when  the  wheels  are  locked  or  fastened,  so  that  thev 


Friction  or  Attrition*  53 

drag  upon  the  surface  of  the  road;  it  is,  therefore,  to 
increase  the  resistance  by  augmenting  the  friction, 
that  a  wheel  is  locked  in  descending  steep  hills, 
where  the  relative  gravitation  gives  too  much  velo- 
city to  the  carriage. 

Levers,  axes,  pullies,  wedges,  screws,  and,  in 
short,  every  mechanic  power,  or  any  description  of 
body,  is  considerably  retarded  by  friction,  when  it 
acts  upon  another  body,  either  in  motion  or  at  rest. 

Friction  is  considered  as  an  uniformly  retarding 
force  in  hard  bodies,  and  not  subject  to  alter  by  dif- 
ferent degrees  of  velocity ;  it  increases  in  a  less  ratio 
than  the  quantity  of  matter,  or  weight  of  the  body ; 
and  the  smallest  surface,  or  the  fewest  parts  in  con- 
tact, has  the  least  friction,  the  weight  being  the  same. 

The  force  or  power  of  friction  varies,  in  propor- 
tion to  the  different  surfaces  in  contact;  that  is,  ac- 
cordingly as  the  surfaces  are  hard  or  soft,  rough  or 
smooth;  even  the  hardest  bodies  which  have  the 
highest  polish  are  not  free  from  inequalities  on  their 
surface,  which  retard  their  motion  when  they  act 
upon  each  other.  When  polished  iron  and  bell-metal 
are  opposed  to  each  other  in  motion,  they  produce 
less  resistance  than  bodies  in  general ;  but  even  these 
polished  planes  do  not  lose  less  than  an  eighth  of 
their  moving  power,  and  others  not  less  than  one- 
third  of  their  force  by  friction. 

As  the  friction  between  rolling  bodies  is  much 
inferior  to  that  which  is  produced  by  bodies  that 
drag,  the  attrition  of  the  axle  in  the  nave  has  been 
lessened  by  a  contrivance  made  with  a  number  of 
small  whe'els,  which  are  called  friction  rollers;*  these 
are  placed  together  in  a  box,  and  fastened  in  the 
nave,  so  that  the  axle  of  the  carriage  may  rest  upon 
them,  and  they  turn  round  their  own  centres  as  the 


*  This  is  the  invention  of  Mr.  John  Garnett,  now  of  New 
Brunswick,  New- Jersey.  Ed. 

F 


54 


Friction  or  Attrition. 


wheel  continues  its  motion,  a  re- 
presents a  section  of  the  axle,  c  c 
the  nave,  and  b  b  the  friction  rollers 
which  turn  round  their  own  axis  as 
the  wheel  revolves  round  the  axle 
of  the  carriage. 

Cylindrical  and  spherical  rollers 
are  used  with  great  advantage  in 
turning  heavy  bodies,  such  as  the 
top  of  a  windmill  or  the  dome  of  an  observatory  ;  or 
in  moving  large  logs  of  wood  or  blocks  of  stone  from 
one  place  to  another.  The  grand  equestrian  statue 
of  Peter  the  Great,  at  Petersburgh,  was  formed  out 
of  an  immense  block  of  stone,  which  was  brought 
from  a  place  some  miles  distant,  by  rolling  it  along 
the  road  on  iron  balls  laid  on  thick  planks. 


55 


PNEUMATICS. 

Atmospherical  or  common  air,  which  consti- 
tutes the  subject  of  Pneumatics,  is  a  thin  transparent 
and  elastic  fluid,  that  surrounds  the  earth  to  a  conside- 
rable height,  and  revolves  with  it  in  its  diurnal  and 
annual  motion.  Independently  of  light,  heat,  and 
electrical  fluids,  it  may  be  considered  as  the  common 
receptacle  for  the  parts  of  all  those  bodies  that  are 
capable  of  being  volatilized  by  fire,  or  dispersed  by 
putrefaction,  exhalation,  evaporation,  or  any  other 
principle  that  changes  the  fixity  or  state  of  bodies,  in 
animal,  vegetable,  or  mineral  productions. 

But  the  hand  that  formed  this  heterogeneous  com- 
pound, has  likewise  tempered  it  for  the  most  essen- 
tial purposes  of  life,  throughout  the  animal  and  ve- 
getable creation;  so  that  it  may  be  considered  as  one 
of  the  prime  agents  of  nature,  in  constituting  and 
preserving  her  works. 

Air  is  generally  ranked  amongst  fluids,  nor  does  it 
differ  from  them  in  its  general  properties,  except  that 
it  admits  of  indefinite  density  by  increasing  com- 
pression, and  that  it  is  incapable  of  fixity  by  excess 
of  cold.  When  the  particles  of  air  are  acted  upon  by 
some  other  body,  they  move  in  every  direction,  and 
communicate  sound,  odour,  or  effluvia,  to  distances 
proportional  to  the  impulse  of  the  particles. 

.  Weight  and  Pressure  of  the  Air. 

That  air  possesses  weight  is  evident,  both  from 
reason  and  experience,  for  its  corpuscular  particles 
are  affected  by  attraction,  which  causes  them  to  gra- 
vitate toward  the  centre  of  the  earth,  and  to  increase 
in  density  as  they  approach  the  surface.  The  weight 
of  the  atmosphere,  likewise,  compresses  the  animal 


56  Weight  and  Pressure  of  the  Air. 

body,  and  keeps  the  fibres  from  being  forced  out  of 
their  natural  order;  but  as  it  presses  equally  on  every 
part,  we  are  insensible  of  its  effects,  except  it  be  par- 
tially removed,  as  in  the  following  experiment. — 
Place  the  hand  on  the  top  of  a  small  glass,  called  a 
hand-glass,  which  is  open  at  both  ends,  and  stands 
on  the  plate  of  the  airpump;  then  exhaust  the  air 
which  it  contains,  and  the  fibres,  or  fleshy  part  of  the 
hand,  that  cover  the  top,  will  distend,  and  rise  up  in 
the  glass,  with  a  painful  sensation,  which  is  occa- 
sioned by  the  want  of  atmospherical  compression  on 
that  part  of  the  hand  which  covers  the  mouth  of  the 
glass.  Whilst  the  hand  remains  in  this  situation,  the 
weight  of  the  atmosphere  which  presses  on  the  up- 
per surface  will  fix  it  so  firmly  to  the  top  of  the  glass, 
that  it  requires  a  considerable  exertion  to  remove  it. 
If  a  piece  of  thin  glass  be  placed  on  the  top,  instead 
of  the  hand,  w^hen  the  air  is  exhausted,  the  glass  will 
be  broken  by  the  pressure  of  the  atmosphere  on  its 
exterior  surface. 

As  heat  rarefies  the  air,  the  effect  of  the  preceding 
experiment  may  be  produced  with  a  common  wine- 
glass. Put  a  piece  of  burning  paper  into  the  glass, 
and  after  the  air  has  been  considerably  rarefied  by 
the  flame,  place  the  fleshy  part  of  the  hand  evenly  on 
the  mouth  of  the  glass,  and  the  pressure  of  the  atmos- 
phere on  the  exterior  part  will  press  it  so  firmly  to  the 
hand  that  it  will  require  some  force  to  remove  it. 
The  pressure  of  the  air  is  likewise  shown  by  its  effect 
on  the  barometer;  it  causing  the  mercury  to  rise  and 
fall  in  the  tube,  as  the  ^veight  of  the  atmosphere  in- 
creases or  decreases.  For  a  further  account  of  which, 
see  the  article  Barometer. 

The  weight  of  air  has  a  sensible  effect  on  a  fine 
scale-beam:  for  on  weighing  a  glass  bottle  which 
contained  40  cubic  inches,  and  afterwards  exhaust- 
ing the  air  and  weighing  it  again,  it  was  found  to  have 
lost  10  grains  of  its  original  w^eight,  which  is  in  the 


Weight  and  Pressure  of  the  A'tr.  57 

ratio  of  about  8  grains  to  a  pint.  The  quantity  of  air 
exhausted  out  of  the  bottle  was  34  inches;  for,  on 
immersing  the  bottle  in  water,  in  an  inverted  posi- 
tion, the  quantity  that  flowed  in  and  occupied  the 
space  of  the  exhausted  air  weighed  8628  grains, 
which,  being  divided  by  253|,  the  number  of  grains 
in  a  cubic  inch  of  water,  produces  34  inches  for  the 
quantity  of  air  exhausted  out  of  the  bottle.  Thus  it 
is  found  that  the  relative  weight  or  specific  gravity  of 
air  to  water  is  as  10  to  8628,  or  as  1  to  8621. 

With  respect  to  the  variation  of  gravity  which  takes 
place  in  the  atmosphere,  it  seems  to  arise  from  dif- 
ferent degrees  of  heat;  for  in  those  parts  which  lie 
between  the  tropics,  where  the  heat  is  constant  and 
regular,  the  variations  in  the  density  of  the  atmo- 
sphere are  likewise  constant  and  regular,  as  the  baro- 
meter is  observed  to  sink  about  half  an  inch  every 
day  when  the  sun  is  above  the  horizon,  and  to  rise 
again  to  the  same  point  in  the  night.  But  from  the 
tropics  to  the  poles,  the  variation  is  irregular  and  in- 
constant, as  the  mercury  is  almost  perpetually  mov- 
ing in  the  tube  of  the  barometer  from  about  the 
height  of  28  to  31  inches;  which  serves  to  indicate 
the  various  changes  that  are  likely  to  take  place  in  the 
weather.  Why  these  changes  thus  irregularly  hap- 
pen, still  depends  on  vague  supposition;  it  is  ima- 
gined that  the  currents  of  air  which  are  formed  by 
the  inequalities  of  the  earth  as  the  whole  atmosphere 
passes  over  its 'surface,  give  or  receive  various  de- 
grees of  heat  as  the  currents  are  rarefied  or  compress- 
ed, in  passing  by  mountains,  moving  over  plains,  or 
descending  from  different  heights  of  the  atmosphere ; 
the  upper  part  of  which  is  found  to  be  a  *region  of 
perpetual  frost.  On  the  immense  range  of  mountains, 
called  The  Andes,  in  America,  as  well  as  on  many 
others,  there  is  a  very  great  difference  of  climate  even 
at  the  same  time.  As  the  situation  of  one  part  of 
these  mountains  is  almost  under  the  line,  it  rests  its 


58  Elasticity  of  the  Air^  and  Height 

base  on  burning  sands  ;  about  half  way  up  is  a  most 
pleasant  and  temperate  climate,  covering  an  exten- 
sive plain,  on  which  is  built  the  city  of  Quito,  whilst 
the  top  is  covered  with  eternal  snow,  perhaps,  co- 
eval with  the  mountain  itself. 

The  Elasticity  of  the  Air  J  and  the  Height  and  Den- 
sity of  the  Atmosphere, 

If  the  atmosphere  which  surrounds  the  earth  were 
nonelastic,  or  of  uniform  density,  like  water,  or  fluids 
in  general,  its  height  might  be  easily  ascertained  by 
means  of  a  barometer;  for  if  the  density  of  air  be  to 
that  of  quicksilver,  as  1  to  11040,  and  if  a  column  of 
quicksilver  2i  feet  high,  be  equal  in  weight  to  a  co- 
lumn of  the  atmosphere  of  an  equal  base,  the  whole 
height  of  the  atmosphere,  supposing  it  to  be  of  an 
equal  density,  would  be  11040,  multiplied  by  2|,  or 
27600  feet,  which  is  little  more  than  5\  miles ;  but 
the  air  is  found  to  possess  an  elastic  quality,  which 
gives  it  a  density,  in  proportion  to  its  compression, 
and  causes  the  atmosphere  to  extend  to  an  unlimited 
height.  Some  idea  of  the  elasticity  of  air  may  be  con- 
ceived by  compressing  a  sponge  or  piece  of  wool,  the 
ramous  parts  of  which  distend  in  every  direction  as 
the  pressure  decreases. 

It  appears,  from  experiment,  that  the  spaces  which 
air  occupies  when  it  is  compressed  by  different 
weights,  are  reciprocally  proportional  to  the  weights 
themselves ;  for  the  more  the  air  is  compressed,  the 
less  space  it  takes  up. 


D 


and  Density  of  the  Atmosphere,  59 

Pour  a  small  quantity  of  mercury  into  a  bent  pipe 
or  tube  a  e,  and  it  will  rise  to  d  ;  then 
stop  it  up  at  A,  so  that  no  air  may  es- 
cape from  that  part  of  the  tube.  When 
it  is  in  this  situation,  it  is  evident  that 
the  weight  of  a  column  of  the  whole 
atmosphere,  equal  in  diameter  to  the 
width  of  the  tube,  rests  upon  d  the 
surface  of  the  mercury.  Now  fill  d  e  ^ 
with  quicksilver,  till  it  is  equal  in  |f| 
weight  to  a  column  of  the  atmosphere,  |B 
or  about  29  inches  high ;  then  double  i 
the  weight  of  the  atmosphere  rests  on  ^^^^ 
the  mercury  at  d,  which  will  force  the  ^^ 
fluid  to  B,  half  way  up  the  opposite  tube.  From  this 
it  appears,  that  the  space  occupied  by  a  certain  quan- 
tity of  air,  under  different  pressures,  is  reciprocally 
proportional  to  the  force  of  the  pressures.  Therefore 
as  the  pressure  of  the  upper  parts  of  the  atmosphere 
upon  the  lower  becomes  less,  according  to  the  dif- 
ferent heights,  it  must  follow,  that  the  air  in  the 
higher  part  of  the  atmosphere,  where  the  pressure  is 
very  inconsiderable,  may  be  rarefied  to  an  almost 
unlimited  extent. 

Now,  supposing  the  atmosphere  to  diminish  in 
density  exactly  in  proportion  to  the  different  heights, 
the  relative  density  of  the  air  may  be  found  at  any 
given  height.  Thus,  it  is  calculated  that  at  the  height 
of  31  miles,  the  atmosphere  is  about  twice  as  rare  as 
on  the  surface  of  the  earth ;  and  at  7  miles  four  times 
as  rare,  and  so  on  in  proportion,  according  to  the 
following  table. 


60  Elasticity  of  the  Air^  ^c. 

Number  of  times 
Height  in  Miles.  as  rare. 

3i 2 

7 4 

14 16 

21 64 

28 256 

35   -   -   -   -   1024 

42 4096 

49  -  -  -  -  16384 
56  -  -  -  -  65536 
63  -  -  -.  -  262144 
70  .   -   -   -  1048576 

Thus,  pursuing  this  calculation,  it  would  seem  that 
a  cubic  inch  of  the  common  air  which  we  breathe, 
would  be  so  much  rarefied  at  the  height  of  500  miles 
as  to  fill  a  sphere  equal  in  diameter  to  the  orbit  of 
Saturn. 

The  elastic  property  of  air  differs  from  the  elasti- 
city of  bodies  in  general ;  when  solid  bodies  are  com- 
pressed they  have  an  elastic  power,  which  causes 
them  to  resume  the  same  figure  they  possessed  pre- 
vious to  compression:  but,  on  removing  the  pressure 
on  air,  it  will  not  only  resume  its  first  bulk,  but  ex- 
pand to  any  extent  as  we  have  already  described. 

The  elastic  force  of  air  acts  in  right  lines ;  so  that 
when  the  compressive  power  is  removed,  it  diverges 
in  all  directions  as  from  a  common  centre.  This  is 
evident  from  the  small  globules  which  are  formed  on 
the  surface  of  an  egg,  or  any  other  body  immersed  in 
water,  when  the  exterior  air  is  exhausted.  When 
these  globules  first  appear  they  are  exceedingly 
small,  but  as  they  increase  in  bulk  they  still  preserve 
their  spherical  form.  Soap  bubbles,  or  glass  globes, 
are  formed  by  the  elasticity  and  equal  divergency  of 
the  particles  of  air.  Innumerable  instances  may  be 
produced  of  the  elasticity  of  air ;  for  further  obser- 
vations and  experiments  on  which  subject,  as  well  as 
on  the  pressure  of  the  atmosphere,  see  the  various 


Air  pump.  61 

experiments  which  follow  the  description  of  the  air- 
pump. 

Air,  in  its  elementary  principles  seems,  peculiarly 
employed  by  nature,  and  it  appears  to  be  equally 
concerned  in  the  preservation  of  vegetable  and  of 
animal  life ;  yet  such  are  the  wonderful  ways  of  Pro- 
vidence, that  whilst  the  heterogeneous  air  of  the  at- 
mosphere is  absorbed  by  the  plant  for  its  support, 
pure  vital  air  is  returned  from  it;  which,  by  mixing 
again  with  the  impure  air  of  the  atmosphere,  renders 
it  more  useful  for  the  purposes  of  animal  life:  so  that, 
by  this  principle  of  absorption  and  regeneration,  ve- 
getable and  animal  existence  is  preserved,  and  the 
wisdom  and  goodness  of  God  are  made  manifest  in 
his  works.* 

A  considerable  quantity  of  air  is  produced  by  dis- 
tillation, fermentation,  putrefaction,  and  explosion, 
as  well  as  from  various  other  effects.  Some  bodies 
absorb  and  destroy  common  air,  such  as  the  fumes  of 
burning  brimstone,  the  flame  of  a  burning  candle, 
and  the  breath  of  all  animals,  f 

Air  pump* 

This  ingenious  and  useful  machine  was  the  inven- 
tion of  Otto  Guericke,  about  the  year  1654;  but  it 
was  soon  afterwards  greatly  improved  by  Boyle,  and 
lias  since  been  brought  to  a  great  degree  of  perfec- 
tion, by  succeeding  philosophers. 

This  machine  is  of  all  others  the  most  useful  in  the 
prosecution  of  pneumatical  studies:  by  means  of  the 
airpump  we  change  the  ordinary  effect  of  the  atmo- 

*  It  is  only  the  vital  part  of  common  air,  called  oxygen  gas, 
that  is  thus  absorbed.  Ed. 

t  The  aeriform  fluids  produced  by  distillation,  fermentation. 
Sec.  are  not  the  same  with  atmospheric  air.  They  have  peculiar 
properties  according  to  the  manner  in  which  they  are  produced, 
and  are,  by  modern  chymists,  commonly  termed  gases. 

G 


62 


Air  pump. 


sphere;  show  the  state  of  existence  of  bodies  under 
different  modifications  of  air;  and  how  highly  essen- 
tial it  is  to  the  preservation  of  animal  and  vegetable 
life. 

From  the  construction  of  the  airpump,  it  is  impos- 
sible to  exhaust  the  whole  of  the  air,  for  the  effect 
produced  by  the  pump  depends  upon  the  elasticity 
of  the  air  which  is  left  in  the  receiver;  this  opens  the 
lower  valve  of  the  machine,  and  the  air  escapes  into 
the  cylinders ;  therefore,  when  the  air  in  the  recipient 
has  not  sufficient  elasticity  to  force  open  the  valves, 
the  action  of  the  handle  cannot  produce  any  further 
rarefaction. 

The  construction  of  the  airpump  is  as  follows: 
A    and  B   are  two  pj^ 

brass  cylinders, 
which  are  closely 
and  firmly  fastened 
down  to  the  table 
or  base  of  the  ma- 
chine H  I  by  the 
head  c  d,  and  the 
columns  e  f  ;  p  is 
the  receiver,  which 
stands  on  g,  a  brass 
circular  plate ;  this 
plate  has  a  small  hole  in  the  middle,  through  which 
the  air  passes  from  the  recipient  into  a  closed  channel 
made  of  brass,  which  communicates  with  the  cylin- 
ders A  B  :  near  the  bottom  of  each  cylinder  is  a  valve 
or  lid  opening  upwards,  and  above  these  valves  are 
two  others,  which  are  moved  up  and  down  by  the 
toothed  rods  l  m,  that  fall  into  a  toothed  wheel  sunk 
in  the  block  c  d,  to  the  axis  of  which  k  the  handle  is 
fixed. 

On  turning  the  handle  one  of  the  pistons  is  raised 
and  the  other  depressed,  consequently  a  rarefied  space 
is  formed  between  the  upper  and  lower  valve  in  one 


Experiments  on  the  Pressure  of  Air.  63 

cylinder;  then  the  air  which  is  contained  in  the  re- 
ceiver rushes  through  the  conducting  pipe,  and  by- 
its  elasticity  forces  up  the  lower  valve  and  enters  the 
rarefied  part  of  the  cylinder  l  a  ;  then  the  valve  closes, 
which  prevents  the  air  from  returning  again  into  the 
receiver.  When  the  motion  is  reversed,  m  the  other 
piston  ascends,  and  l  is  depressed;  in  its  depression 
the  elasticity  of  the  air,  contained  between  the  two 
valves,  forces  open  the  uppermost  valve,  and  it  es- 
capes into  the  upper  part  of  the  cylinder;  then  the 
valve  closes  again  and  prevents  its  return. 

The  opposite  piston  performs  the  same  operation, 
but  the  motions  are  alternate,  so  that  whilst  one 
piston  exhausts  the  air  from  the  receiver,  the  other 
is  discharging  it  from  the  top  of  the  cylinder.  Thus, 
by  continued  exhaustion,  the  density  of  the  air  keeps 
decreasing  in  the  receiver,  till  its  elasticity  is  no 
longer  able  to  force  up  the  lower  valves,  which  ter- 
minates the  effect  of  the  machine.  When  the  expe- 
riment is  performed,  the  air  is  again  admitted  into  the 
recipient,  by  unscrewing  a  small  nut  at  q^  which  com- 
municates with  the  air  channel,  and  restores  an  equi- 
librium to  the  opposite  sides  of  the  receiver.  In  the 
base  of  the  machine  is  a  small  hole,  which  enters  the 
air-pipe,  and  over  this  is  placed  a  quicksilver  gage 
and  a  small  receiver,  to  show  the  different  densities 
of  the  air  in  the  recipient  when  the  machine  is  at 
work. 

Experiments  on  the  Pressure  of  Air, 

It  has  been  already  stated,  that  our  bodies  sustain 
great  atmospherical  pressure,  which  may  be  made 
still  more  evident  by  its  effects  on  the  exterior  part 
of  an  exhausted  receiver.    When  it  is  first  placed 


64 


Expemyients  on  the  Pressure  of  Air, 


upon  the  plate  of  the  airpump,  the 
force  or  pressure  of  the  air  which  is 
contained  in  the  receiver,  being  equi- 
valent to  that  which  acts  on  the  exte- 
rior part,  it  may,  like  our  bodies,  be 
moved  with  facility :  but  as  the  air  is 
exhausted,  the  equilibrium  is  destroy- 
ed between  the  interior  and  exterior 
surfaces,  till  the  pressure  of  the  atmo- 
sphere on  the  outside  fixes  the  receiver  so  firmly  to 
the  plate,  that  it  requires  a  greater  force  than  that  of 
one  man  to  remove  it.  In  treating  of  the  barometer 
it  is  there  shown,  that  the  atmosphere  presses  with  a 
weight  of  \5lbs,  upon  every  square  inch  of  the  earth's 
surface;  therefore,  supposing  the  surface  of  the  re- 
ceiver to  contain  only  36  square  inches,  the  pressure 
of  the  atmosphere  upon  it  is  36  multiplied  by  15,  or 
54^0lbs.  weight;  but  some  allowance  must  be  made 
for  imperfect  exhaustion. 

The  rising  and  failing  of  the  mercury  in  the  pump 
gage  shows  the  different  degrees  of  density  in  the 
receiver. 

B  is  a  small  tube  filled  with  quicksilver, 
and  immersed  in  the  cup  a,  which  likewise 
contains  any  given  quantity  of  mercury: 
these  are  placed  under  c,  a  small  receiver, 
which  is  set  over  the  aperture  of  the  air 
channel  in  the  block  of  the  machine.  Now, 
before  the  exhaustion  takes  place,  the  mer- 
cury is  supported  in  the  tube  b,  by  the 
pressure  on  the  surface  at  a  :  but  as  the  air 
is  withdrawn  by  the  operation  of  the  pump, 
the  pressure  on  the  surface  of  the  mercury 
decreases  as  the  density  decreases,  consequently  that 
which  stands  in  the  tube  loses  part  of  the  support 
that  sustains  it,  and  descends  into  the  cup,  in  propor- 
tion as  the  air  is  exhausted,  which  serves  to  show  the 
density  of  the  air  that  remains  in  the  receiver. 


6S 


The  Pressure  of  Air  on  the  Bladder -glass. 

This  glass,  which  is  open  at  both 
ends,  has  the  wider  end  covered  with  a 
piece  of  wet  bladder,  and  then  left  to 
dry ;  after  it  is  perfectly  dry  the  open 
end  is  placed  on  the  pump  plate,  and  the 
interior  air  is  exhausted,  till  the  weight 
of  the  air  on  the  top  of  the  bladder  bursts  it  with  a 
considerable  report.  If  the  air  be  exhausted  out  of  a 
thin  square  glass  bottle  the  exterior  pressure  will 
break  it  to  pieces. 

A  pleasing  experiment,  and  a  demonstrative  proof 
of  the  gravity  and  pressure  of  the  atmosphere,  is 
shown  by  what  is  usually  called  the  Magdeburg  He- 
mispheres. 

This  machine  is  a  hollow  globe  of  brass, 
divided  into  two  hemispheres  a  b;  in  the 
lower  part  c,  are  a  stop-cock  and  a  tube 
which  screws  into  the  pump-plate.  Be- 
fore the  air  is  exhausted  from  the  interior 
of  the  globe,  the  interior  and  exterior 
pressures  are  equal,  so  that  the  parts  may 
be  separated  with  the  greatest  facility ;  but 
when  the  counterbalancing  force  is  re- 
moved from  the  interior,  and  the  stop- cock  is  shut 
to  prevent  the  return  of  the  air,  the  pressure  of  the 
atmosphere  on  the  surface  will  compress  the  two  parts 
of  the  sphere  so  closely  together  that  it  will  require 
more  than  an  ordinary  force  to  pull  them  asunder. 

By  means  of  this  experiment  we  are  practically  able 
to  determine  the  actual  pressure  of  the  atmosphere 
on  any  given  surface :  for  suppose  that  the  mouth  of 
the  hemispheres  contains  12  square  inches,  and  that 
by  a  steelyard  hooked  to  the  ring  at  the  top  it  requires 
X^Olbs,  weight  to  separate  the  two  parts;  then  if  180 
be  divided  bv  12,  the  result  is  15 lb,   which  is  the 


66 


Magdeburg  Hemispheres. 


pressure  on  a  square  inch  of  the  surface.  This  atmo- 
spherical pressure  on  bodies  is  confirmed  by  another 
experiment,  under  the  head  of  Barometer.  If  these 
hemispheres  be  placed  under  an  exhausted  receiver, 
so  that  the  pressure  of  the  air  on  both  sides  of  the  ma- 
chine be  made  equal,  they  will  separate  with  the 
greatest  ease ;  which  is  an  additional  proof  that  the 
pressure  of  the  atmosphere  alone  holds  them  together. 

The  fountain  in  vacuo  is  an  entertaining  experi- 
ment, which  shows  the  force  of  atmospheric  pressure 
on  the  surface  of  bodies. 

A  tall  receiver  is  placed  over  a  brass  plate  and  jet- 
pipe  ;  these  are  connected  with  the  air- 
pump,  by  means  of  a  stop- cock  and  tube: 
after  the  air  is  exhausted  out  of  the  re- 
ceiver, the  cock  is  shut  to  prevent  its  re- 
turn; then  the  whole  is  unscrewed  from 
the  plate  of  the  receiver,  and  the  lower 
end  of  the  tube  is  immersed  in  a  vessel  of 
water :  on  opening  the  stop- cock,  the 
pressure  of  the  atmosphere  on  the  surface 
of  the  water  in  the  vessel  having  no  coun- 
terpoise from  the  interior  of  the  cylinder, 
forces  up  the  fluid  through  the  jet-pipe 
with  considerable  velocity,  which  forms  a  pleasing 
jet  d'eau  or  fountain  in  vacuo. 


v^PW^pW'W 


67 


Experiments  on  the  Elasticity  of  Air. 

If  a  glass  bottle,  with  a  small  tube  in  its 
neck,  be  half  filled  with  water,  and  placed 
under  a  receiver;  when  the  air  is  exhaust- 
ed out  of  the  recipient,  that  which  is  con- 
tained in  the  bottle  loses  its  counter-pres- 
sure, distends  by  the  elasticity  of  its  parts, 
presses  on  the  surface  of  the  water,  and 
forces  the  fluid  through  the  neck  of  the 
bottle,  to  a  considerable  height. 

The  power  of  a  fountain  of  this  kind 
may  be  greatly  augmented,  even  without 
placing  it  under  an  exhausted  receiver, 
by  forming  the  vessel  of  brass  instead  of  glass,  and 
by  using  an  injecting  machine  to  compress  and  con- 
dense the  air  in  the  upper  part  of  the  globe  ;  for  if  the 
air  be  compressed  in  a  brass  globe,  the  sides  of  which 
are  capable  of  resisting  the  expansive  force  of  the  in- 
jected air,  the  height  to  which  the  water  ascends  will 
be  proportional  to  the  density  or  elasticity  of  the  air 
that  presses  on  the  surface  of  the  fluid:  now,  as  the 
density  may  be  made  many  times  greater  than  that 
of  the  atmosphere,  the  elastic  power  which  arises 
from  the  compression  will  overcome  the  resistance  of 
the  atmosphere,  and  the  water  will  spout  out  of  the 
neck  with  a  force  proportional  to  the  difterence  of  the 
densities  between  the  internal  and  external  air. 

Those  who  have  not  the  advantage  of  a  complete 
apparatus,  may,  by  the  humble  means  of  a  phial  and 
small  tubfe,  or  a  tobacco-pipe,  produce  a  sufficient 
effect  to  satisfy  themselves  of  the  elasticity  of  air. 

Fill  a  phial  about  half  full  of  water,  insert  one  end 
of  the  pipe  in  the  ffuid,  and  let  the  other  project  about 
an  inch  above  the  neck  of  the  bottle;  then  close  up 
the  pipe  in  the  neck  with  sealing-wax,  so  that  the  air 
may  not  escape  from  the  bottle.  After  the  machine  is 


68  Experiments  on  the 

completed,  blow  strongly  through  the  tube,  and  the 
elasticity  of  the  air,  which  is  compressed  in  the  upper 
part  of  the  bottle,  will  so  far  overcome  the  resistance 
of  the  atmosphere  or  exterior  air,  as  to  force  the  wa- 
ter out  of  the  pipe  some  inches  in  height,  till  the  den- 
sity of  the  interior  and  exterior  air  become  equal. 
When  the  water  is  exhausted  below  the  end  of  the 
pipe  in  the  bottle,  it  may  be  supplied  by  sucking  the 
tube  with  the  lips,  and  instantly  stopping  the  aper- 
ture of  the  pipe  with  the  finger;  then  immerse  the 
end  in  a  basin  of  water,  and  when  the  finger  is  remov- 
ed it  will  flow  into  the  bottle.  For  as  part  of  the  air 
has  been  drawn  out  of  the  phial  by  the  lips,  that  which 
remains  is  less  dense  than  the  exterior  air;  so  that 
the  pressure  on  the  surface  of  the  water  in  the  basin 
overcomes  the  resistance  of  the  rarefied  air  in  the 
bottle,  and  forces  the  fluid  up  the  pipe,  till  the  gravi- 
ties of  the  interior  and  exterior  air  become  equal. 
As  heat  distends  the  volume  of  air,  by  imposing  a 
superior  degree  of  elasticity ;  if  the  phial  be  held  near 
the  fire,  or  even  warmed  by  the  heat  of  the  hand,  this 
will  increase  the  elastic  force  of  the  air,  and  cause  a 
small  discharge  of  water  from  the  neck  of  the  tube. 

If  a  very  small  quantity  of  air  be  tied  up  in  a  blad- 
der, when  it  is  placed  under  a  receiver,  the  sides  of 
the  bladder  will  gradually  distend,  as  the  exterior  air 
is  decreased,  till  it  is  completely  inflated  by  the  elas- 
ticity of  the  air  which  it  contains.  If  a  superior  quan- 
tity of  air  be  left  in  the  bladder,  when  the  receiver  is 
exhausted,  the  expansive  force  or  elastic  power  of 
the  air  which  is  tied  up  will  burst  it  with  a  conside- 
rable report. 

The  elastic  power  of  air  may  be  further  shown  by 
what  is  called  the  bladder  and  weights. 


Elasticity  of  the  Air.  69 

The  bladder  a  contains  a  small  quan- 
tity of  air,  and  is  placed  in  the  frame  b, 
with  the  weight  c  laid  upon  it.  On  ex- 
hausting the  air  out  of  the  receiver,  the 
small  quantity  which  is  contained  in  the 
bladder  will  distend  with  such  force,  by 
its  elasticity,  as  to  raise  up  the  weights 
that  are  laid  upon  it.  By  injecting  air 
into  cased  bladders,  with  a  forcing  piston, 
any  quantity  of  power  may  be  obtained 
for  raising  considerable  weights. 

Bodies  in  general  contain  a  quantity  of  air;  parti- 
cularly wood,  fruits,  and  other  vegetables.  If  a  shri- 
velled apple  be  placed  under  an  exhausted  receiver, 
it  will  be  plumped  out,  and  appear  quite  fresh,  by  the 
spring  of  the  air  which  is  contained  under  its  skin. 
If  the  apple  be  immersed  in  water,  under  an  exhaust- 
ed receiver,  part  of  the  air  which  it  contains  will  issue 
in  small  globules  from  every  part  of  its  surface.  The 
air  which  is  contained  in  wood  or  the  pores  of  bo- 
dies in  general,  may  be  seen  by  immersing  them  in 
a  similar  manner.  From  the  porosity  of  an  egg-shell, 
the  included  air  that  is  forced  from  the  ^g^  through 
the  shell  by  the  elasticity  of  the  air,  will  form  itself 
into  beautiful  pearly  globules  all  over  the  surface : 
these  globules  of  air  are  driven  in  again  when  the  air 
is  admitted  into  the  receiver. 

The  expansion  or  elasticity  of  air  diminishes  the 
specific  gravity  of  bodies. 

Take  a  small  bladder  which  contains  a  portion  of 
air,  and  sink  it  with  a  leaden  weight  in  a  vessel  of  wa- 
ter; then  place  it  under  an  exhausted  receiver,  and 
the  elasticity  or  dilatation  of  the  air,  which  is  con- 
tained in  the  bladder,  will  raise  it  and  the  weight  to 
the  surface  of  the  water.  Balloons  ascend  by  a  similar 
principle ;  for  the  gravity  of  the  air  which  they  con- 
tain being  less  than  that  of  the  atmosphere,  they  rise 
in  proportion  to  the  difference. 

H 


70 


Cllt'j^^i*-*' 


Miscellan eo us  Exp  erim  en ts. 

The  porosity  of  wood  may  be  shown 
in  the  following  manner:  Let  a  solid  piece 
of  wood  be  fixed  to  the  bottom  of  a  cup, 
and  passed,  air-tight,  into  the  neck  of  the 
bottle ;  when  mercury  is  poured  into  the 
cup,  and  the  air  exhausted  out  of  the 
bottle,  the  pressure  of  the  atmosphere  on 
the  surface  of  the  mercury  in  the  cup 
will  force  it  through  the  pores  of  the  so- 
lid piece  of  wood,  and  it  will  fall  like  a 
silver  shower,  to  the  bottom  of  the  bottle. 

It  has  already  been  observed,  that  air  is  produced 
by  fermentation ;  this  will  account  for  that  frothy  and 
foaming  appearance  which  we  observe  when  a  cork 
is  drawn  out  of  a  bottle  of  ale,  beer,  perry,  or  cider, 
and  some  kinds  of  wine ;  for  when  the  liquor  is  first 
put  into  the  bottle,  it  carries  with  it  a  considerable 
portion  of  air,  this  becomes  greatly  augmented  by 
fermentation,  and  escapes  rapidly  from  its  elasticity 
when  the  cork  is  drawn  out.  The  different  appear- 
ances of  this  evaporation  proceed  from  the  different 
qualities  of  the  liquor;  for  in  glutinous  bodies,  like 
ale,  the  air  is  not  able  to  disengage  itself  freely  from 
the  fluid,  and  it  falls  over  the  neck  of  the  bottle  in 
froth,  but  it  escapes  more  directly  from  those  that  are 
thinner,  such  as  wine,  perry,  8cc. 

Air  is  absolutely  necessary  for  the  existence  of  fire 
and  flame;  for  if  a  lighted  candle,  or  a  piece  of  burn- 
ing wood,  be  placed  under  a  receiver,  it  will  be  ex- 
tinguished as  soon  as  the  air  is  exhausted.  When 
gunpowder  is  fired  under  an  exhausted  receiver,  it 
will  not  explode,  but  melts  and  dies  away  without 
any  report."^' 

*  Gunpowder  containing  oxygen  in  its  composition,  may  be 
fired  in  vacuo  ;  though  the  explosion  will  be  much  less  audible 
1  ban  in  tlw  open  air. "  Ed. 


Miscellaneous  Experiments.  71 

As  sound  is  propagated  by  pulses  of  air,  it  is  not 
audible  under  an  exhausted  receiver.  For  if  a  bell  be 
struck  in  vacuo,  we  are  insensible  of  any  vibratory 
effect,  but  as  the  air  is  admitted,  the  sound  is  aug- 
mented in  proportion ;  so  that,  if  the  density  of  the  air 
in  the  receiver  were  increased  beyond  that  of  the  at- 
mosphere, the  sound^of  a  bell  would  be  more  forcibly 
heard  in  such  a  situation  than  when  it  is  struck  in  the 
open  air. 

The  time  of  descent  of  light  and  heavy  bodies  in 
vacuo  is  always  the  same.  For  the  difference  of  time 
which  we  observe  in  the  open  air  proceeds  from  the 
resistance  of  the  medium  through  which  they  de- 
scend; but  as  this  is  nearly  removed  in  an  exhausted 
receiver,  a  feather  will  fall  with  the  same  velocity  as 
a  guinea. 

Bodies  which  balance  each  other  in  the  open  air, 
lose  their  equilibrium  in  vacuo.  If  a  piece  of  cork  and 
a  piece  of  lead,  which  balance  each  other  in  air,  be 
weighed  again  under  an  exhausted  receiver,  the  end 
that  suspends  the  cork  will  descend,  for  when  both 
these  bodies  are  weighed  in  air,  they  lose  the  weight 
of  an  equal  bulk  of  the  air,  consequently  the  cork 
loses  more  weight  than  the  lead;  but  when  they  are 
placed  under  an  exhausted  receiver,  what  the  cork 
lost  by  its  magnitude  in  the  open  air  it  now  gains  in 
vacuo;  and  as  the  bulk  of  the  lead  is  much  less  than 
the  bulk  of  the  cork,  the  weight  of  the  cork  in  vacuo 
will  exceed  the  weight  of  the  lead  as  much  as  their 
respective  bulks  of  air  exceed  each  other  in  weight. 

The  rise  of  vapour  and  smoke  is  caused  by  the 
density  of  the  air;  for  if  smoke,  or  vapour,  be  placed 
under  an  unexhausted  receiver,  it  will  rise  and  darken 
the  interior;  but  as  the  air  is  exhausted  the  smoke 
descends,  and  at  length  leaves  the  vessel  quite  clear. 
This  serves  to  show  that  the  air  is  lightest  in  moist 
and  hazy  weather,  for  then  the  density  of  the  atmo- 
sphere is  not  sufficient  to  support  the  humidity  it 


72  Miscellaneous  Experiments. 

contains;  therefore  the  weight  of  the  vapour  over- 
powers the  resistance,  and  it  descends  in  aqueous 
particles.  Winged  animals  are  incapable  of  flight  in 
vacuo.  If  a  butterfly  be  suspended  by  its  horns,  from 
a  thread  in  the  middle  of  the  receiver,  before  it  is  ex- 
hausted the  insect  will  fly  with  apparent  ease  from  one 
side  to  the  other ;  but  when  the  air  is  withdrawn,  it 
hangs  perpendicularly,  and,  notwithstanding  its  ef- 
forts, it  is  unable  to  change  its  position. 

Breathing,  which  is  the  principle  action  of  life, 
arises  from  the  compression  and  elasticity  of  air. 

The  air  which  we  breathe  is  compressed  by  the  act 
of  inspiration,  and  the  vital  principle  of  our  being  is 
supported  by  the  wholesome  particles  that  we  inhale, 
whilst  those  which  are  corrupted  by  our  lungs  and 
unfit  for  the  purpose  of  animal  life  are  discharged  in 
the  act  of  expiration. 

When  animals  are  placed  under  a  receiver  and  pre- 
cluded from  the  common  air  they  soon  lose  their  life; 
though  this  effect  is  not  immediate,  but  in  proportion 
to  the  nature  of  the  animal  and  the  quantity  of  pure 
air  left  in  the  receiver.  Some  animals,  such  as  a  toad 
or  a  snake,  will  exist  for  a  considerable  time;  but 
when  the  remaining  air  is  completely  corrupted  the 
animal  dies. 

The  quantity  of  wholesome  common  air  which  is 
necessary  for  a  man's  existence  is  about  one  gallon  a 
minute :  this  destroys  the  vital  quality  of  about  23 
hogsheads  in  24  hours.  A  burning  candle  will  con- 
sume nearly  the  same  quantity  in  the  same  time.  In- 
haling air  which  is  deprived  of  its  oxygen  by  passing 
through  fire,  or  a  heated  tube,  causes  immediate 
death.  Animals  placed  under  a  receiver,  which  is 
supplied  with  burnt  air,  expire  instantly. 

As  the  existence  of  the  animal  body  depends  on  a 
proper  supply  of  fresh  air,  some  idea  of  its  operation 
in  the  lungs  may  be  formed  by  the  following  experi- 
ment 


Miscellaneous  Experiments.  73 

If  a  small  bladder  be  tied  to  a  pipe  and  thrust  into 
a  bottle,  so  as  to  be  air-tight  in  the  neck,  it  may  re- 
present the  lungs  in  the  thorax,  and  the  hollow  tube 
the  trachea  or  windpipe :  then  the  air  which  is  con- 
tained in  the  bottle,  about  the  bladder,  shows  the  air 
in  the  breast  that  compresses  the  lungs,  which  is  ba- 
lanced by  inspiration,  or  the  air  we  inhale.  When 
this  is  placed  under  a  receiver,  it  shows  the  natural 
state  of  the  lungs :  but  as  the  air  in  the  receiver  be- 
comes exhausted,  the  elasticity  of  the  air  in  the  bot- 
tle begins  to  compress  the  bladder,  till  at  length  its 
sides  are  joined  closely  together. 

The  lungs  of  animals  are  compressed  in  the  same 
manner  when  they  are  deprived  of  the  counterbalanc- 
ing force  of  the  interior  air;  this  produces  a  violent 
sensation  or  pressure  on  the  breast,  and  stops  the  cir- 
culation of  the  blood  in  the  lungs,  which  causes  suf- 
focation and  death,  unless  the  air  be  speedily  admit- 
ted; then,  if  life  be  not  too  far  exhausted,  the  sensa- 
tion decreases,  the  action  and  reaction  in  the  lungs 
become  equal,  and  the  animal  recovers. 

Having  shown  the  different  properties  of  air,  by- 
some  of  the  most  familiar  experiments,  we  will  pro- 
ceed to  an  explanation  of  the  construction  and  use  of 
some  of  the  principal  pneumatical  instruments  and 
machines. 

Barometer. 

After  the  gravitating  force  or  pressure  of  the 
atmosphere  was  discovered  by  Galileo,  it  was  found 
by  experiment,  that  water  might  be  raised  by  a  com- 
mon pump  to  a  certain  height  and  no  further.  He- 
then  happily  imagined  that  this  limited  ascent  must 
be  the  counterbalancing  power  to  an  equal  column  of 
the  atmosphere.  This  idea  was  seized  and  improved 
upon  by  Torricelli,  Pascal,  and  some  others ;  who 
considered,  that  if  the  weight  of  a  column  of  water  34 


74 


Barometer, 


feet  high,  which  is  about  the  height  it  ascends  in  va- 
cuo, be  equal  to  a  cokimn  of  the  atmosphere  of  the 
same  base ;  any  other  fluid,  differing  in  specific  gra- 
vity from  water,  would  balance  the  same  column  of 
the  atmosphere,  by  rising  to  a  proportionate  height: 
and  as  quicksilver  is  about  fourteen  times  heavier 
than  water,  it  was  supposed  that  a  column  of  mercury 
of  about  the  fourteenth  part  of  the  height  of  a  column 
of  water,  would  counterbalance  the  air  like  the  co- 
lumn of  water.  The  experiment  equalled  the  wishes 
of  the  experimentalists;  and,  after  filling  a  glass  tube 
with  mercury,  which  was  closed  at  the  upper  end  to 
keep  off  the  weight  of  the  atmosphere,  they  found  the 
mercury  fluctuated,  at  different  times,  between  the 
heights  of  28  and  31  inches:  this  variation  was  disco- 
vered to  arise  from  the  different  pressure  of  the  atmo- 
sphere, which  varied  according  to  the  different  states 
of  the  weather.  Thus  a  pneumatical  instrument  was 
formed,  which  has  equalled,  if  not  surpassed,  any 
other  in  promoting  physical  knowledge.  The  baro- 
meter is  variously  constructed,  for  the  purpose  of 
accuracy  and  convenience.  In  the  following  descrip- 
tions we  have  noticed  those  which  are  in  the  most 
common  use. 

If  A  B,  be  a  small  glass  tube 
about  34  inches  long,  and  a  quar- 
ter of  an  inch  in  diameter,  closed 
at  one  end;  when  this  tube  is 
filled  with  mercury  that  has  been 
thoroughly  freed  from  air,-*-  and 
inverted  in  the  basin,  the  mer- 
cury will  sink  down  in  the  tube 
to  the  point  d,  somewhere  be- 
tween 28  and  31  inches  from  the 
surface  of  that  which  is  in  the 
basin,  leaving  a  vacuum  in  the 
upper  part  of  the  tube:  so  that 

*  By  boiling  it  in  the  tube.  Ed. 


hA 


Barometer.  75 

this  part  opposes  no  resistance  to  the  rising  and  falling 
of  the  upper  surface  of  the  mercury,  and  leaves  the 
free  action  of  the  atmosphere  to  press  on  the  lower, 
without  any  counterbalancing  force;  which  causes 
the  mercury  to  rise  in  the  tube  as  the  density  of  the 
atmosphere  increases,  and  to  sink,  as  the  pressure 
decreases  on  the  surface  in  the  cup. 

Barometers  are  usually  made  of  a  tube,  with  a 
curved  neck  and  bulb,  being  more  commodious  than 
the  basin  and  tube.  This  is  fastened  to  a  frame, 
which  has  a  scale  of  equal  parts  placed  between  28 
and  31  inches  from  the  surface  of  the  mercury,  being 
the  extreme  variation  of  atmospherical  pressure:  and 
this  scale  likewise  contains  a  prognostic  state  of  the 
weather  against  different  heights  of  the  mercury,  with 
a  moving  index  and  nonius  to  determine  the  changes, 
either  in  rising  or  falling. 

To  make  these  barometers  tolerably  exact,  the 
circular  area  of  the  bulb  should  be  at  least  30  or  40 
times  larger  than  that  of  the  tube,  so  that  the  mer- 
cury may  be  as  little  affected  as  possible  whilst  it 
rises  and  falls ;  if  the  mercury  were  to  rise  half  an  inch 
in  the  tube  and  fall  a  quarter  of  an  inch  in  the  bulb, 
the  difference  of  the  height  would  be  three  quarters 
of  an  inch;  for  the  height  of  the  column  is  taken  from 
the  surface  of  the  mercury  in  the  bulb  to  its  height 
in  the  tube.  Therefore,  according  to  the  above  varia- 
tion, the  surface  of  the  mercury  against  the  scale  at 
the  top  of  the  barometer,  would  be  a  quarter  of  an 
inch  short  of  its  due  height;  but  by  increasing  the 
circular  area  of  the  bulb  to  forty  times  that  of  the 
tube,  the  rising  or  depression  of  the  mercury  in  the 
bulb  at  the  greatest  variation  in  the  tube  would  not 
be  more  than  the  tenth  or  twelfth  part  of  an  inch, 
which  makes  but  an  inconsiderable  difference  in  the 
accuracy  of  the  instrument. 


76 


Diagonal  Barometer, 

The  inflected  or  diagonal  barometer,  which  was  in- 
vented  by  Moreland,  is  nothing  more  y. 
than  the  common  barometer  with  the 
upper  part  of  the  tube  bent  into  an 
obtuse  angle  at  b  ;  the  line  a  c,  which  (;^[_ 
is  about  three  inches  long,  is  equal 
to  the  perpendicular  height  of  that 
part  of  the  instrument,  which  is  partly 
empty  in  those  tubes  that  are  straight, 
when  the  atmosphere  is  in  a  rarefied 
state;  and  as  a  b  is  longer  than  a  c, 
the  mercury  must  pass  through  a 
greater  space  in  attaining  the  same 
height,  consequently  the  variation, 
either  in  rising  or  falling,  is  more 
minutely  determined. 

There  are  many  other  kinds  of 
barometers,  the  construction  of  which  renders  them 
less  perfect  than  the  preceding;  therefore,  as  their 
description  would  be  more  curious  than  useful,  they 
have  been  omitted. 

Pressure  of  the  Atmosphere^  according  to  the 
Barometer. 

If  a  common  barometer  be  placed  under  a  cylindric 
receiver  on  the  plate  of  the  airpump,  the  mercury 
that  stands  in  the  tube  will  descend  into  the  bulb,  in 
proportion  to  the  quantity  of  air  which  is  exhausted 
out  of  the  receiver,  till  the  tube  is  completely  evacua- 
ted. This  will  rise  again  in  proportion  to  the  quantity 
of  air  which  is  admitted  into  the  receiver;  which  evi- 
dently shows,  that  the  counterpressure  of  the  air  sup- 
ports the  mercury  in  the  tube,  and  that  the  variation 
in  the  height  of  the  column  is  caused  by  the  different 


Pressure  of  the  Atmosphere,  77 

degrees  of  pressure  on  the  surface  of  the  mercury 
which  is  contained  in  the  bulb. 

As  the  atmospherical  variation  is  contained  be- 
tween 28  and  31  inches,  a  mean  distance  of  291 
inches  may  be  taken  as  the  height  and  weight  of  a 
cokimn  of  mercury,  which  is  equal  to  a  column  of 
the  atmosphere  of  the  same  base  in  a  medium  state. 
Now,  as  a  cubic  inch  of  mercury  weighs  about  eight 
ounces  avoirdupois,  a  square  pillar  of  mercury  W\ 
inches  in  height,  the  base  of  which  is  an  inch  square, 
would  weigh  \Slhs.  nearly,  which  are  equal  to  the 
pressure  of  the  atmosphere  on  a  square  inch  of  sur- 
face: these  multiplied  by  144,  give  2160/^^.  for  the 
pressure  on  a  square  foot.  Then,  as  the  number  of 
square  feet  on  the  surface  of  a  middle  sized  man  is 
computed  at  14i,  this  multiplied  by  2160,  produces 
31320/^^.,  or  nearly  14  tons  weight,  which  perpe- 
tually presses  on  our  bodies. 

We  may,  by  an  easy  calculation  find  the  pressure 
of  the  atmosphere  on  the  whole  surface  of  the  earth, 
which  is  several  millions  of  millions  of  tons. 

It  may  be  asked,  why  we  are  not  sensible  of  the 
great  pressure  of  the  atmosphere  upon  our  bodies  ? 
To  which  it  may  be  answered,  that  the  pressure  is 
counterbalanced  by  the  air  within  us,  which  makes 
the  sensation  negative,  and  leaves  us  no  more  idea  of 
the  weight  than  we  have  of  the  motion  of  the  earth, 
or  of  a  ship  on  a  smooth  sea,  when  we  are  sailing  at 
a  great  distance  from  the  shore.  But  if  this  pressure 
be  partially  removed  from  any  part  of  our  bodies,  we 
become  sensible  of  it;  for,  as  the  fibres  are  greatly 
distended  and  moved  out  of  their  natural  order,  it 
produces  considerable  pain  in  the  part  which  is  un- 
compressed. 

Even  the  variation  of  the  atmosphere  generally 
produces  a  change  in  bodily  sensation,  which  makes 
invalids  very  sensible  of  the  alteration  without  con- 
sulting their  weatherglass. 


78  Thermometer, 

The  different  densities  of  the  atmosphere  are  of  the 
greatest  consequence  to  heahh;  for  in  dull  hazy 
"sveather,  when  the  air  is  in  its  lightest  state,  the  want 
of  suflicient  compression  on  the  body  produces  pains 
in  the  breast,  and  a  difficulty  in  breathing,  which  par- 
ticularly affects  those  who  have  asthmatical  com- 
plaints: it  likewise  suffers  a  distention  of  the  various 
vessels  in  our  bodies,  which  causes  the  circulation  of 
the  blood  to  lose  part  of  its  activity,  and  thus  the 
whole  system  is  oppressed  with  pain,  languor,  and 
debility.  Mistaking  causes  for  effects,  it  is  a  vulgar 
opinion  that  in  dull  hazy  weather  the  air  is  in  its 
heaviest  state,  but  the  reverse  is  the  fact;  for  then  the 
air  is  the  lightest,  which  suffers  the  humid  particles 
that  float  in  it  to  descend  to  the  earth. 

In  clear  and  serene  weather  the  atmospheric  pres- 
sure is  the  greatest,  which  braces  or  constringes  our 
bodies,  and  gives  such  force  to  the  circulation  of  the 
blood  as  removes  obstructions  in  the  vessels  and 
produces  a  proper  tone  in  the  system;  thus  tending 
to  promote  the  blessing  of  health  and  the  enjoyment 
of  life. 

Thermometer. 

The  thermometer  is  a  small  tube  and  bulb  filled 
with  a  fluid,  and  is  used  to  determine  the  various 
degrees  of  heat  in  the  atmosphere,  water,  spirit,  &c. 
,  We  find,  by  experiment  that  air  and  all  kinds  of  fluids 
contract  and  expand  by  different  degrees  of  heat; 
therefore,  if  any  fluid  l3ody  be  enclosed  in  a  small 
glass  tube  purged  from  aii',  it  will  sink  or  contract 
from  exterior  frigidity,  and  rise  or  expand  on  the 
increase  of  heat. 

This  tube  of  the  instrument  is  generally  filled  with 
spirits  of  wine,  linseed  oil,  or  quicksilver.  Spirits  of 
wine,  being  very  limpid,  is  very  useful  in  observa- 
tions on  air,  where  there  is  no  great  difference  of 
lieat;  but  if  the  heat  be  too  violent,  the  expansion  of 


Thermometer, 


79 


the  spirit  will  burst  the  tube;  and  if  the  cold  be  ex- 
treme the  spirit  will  freeze;  consequently,  in  either 
case,  the  instrument  is  useless.  To  remedy  the  first 
of  these  inconveniences  Newton  filled  his  thermome- 
ter with  linseed  oil;  which  requires  four  times  the 
heat  of  boiling  water  to  cause  ebullition:  this  was 
found  to  succeed  very  well  in  experiments  of  heat, 
but  in  those  of  frigidity  it  is  much  more  susceptible 
of  cold  than  the  spirit  thermometer.  To  meet  both 
sides  of  the  difficulty,  Fahrenheit  made  one  with 
quicksilver,  which  preserves  its  effect  in  either  ex- 
treme. 

When  this  instrument  is  constructed,  the  bulb  and 
tube  are  filled  with  a  certain  quantity  of 
mercury  that  has  been  thoroughly  purged 
from  air  or  moisture  by  boiling;  and  before 
the  end  of  the  tube  is  hermetically  sealed, 
or  melted  and  closed,  the  air  is  entirely  ex- 
pelled from  it  by  heating  the  mercury, 
w^hich  rarefies  it,  and  drives  it  to  the  top 
of  the  tube. 

A  scale  is  annexed  to  the  side  of  the 
tube,  which  Fahrenheit  divided  into  600 
parts,  beginning  from  the  point  (0),  the 
greatest  degree  of  cold  which  could  be  produced  by 
surrounding  the  bulb  of  the  thermometer  with  a  mix- 
ture of  snow  or  pounded  ice,  with  sal  ammoniac  or 
sea  salt.  The  point  at  which  mercury  begins  to  boil 
he  laid  down  as  the  greatest  degree  of  heat,  and  the 
intermediate  distance  was  divided  into  600  parts  to 
show  the  various  degrees  of  heat  between  the  two 
extremes.  When  this  thermometer  was  immersed  in 
water  just  beginning  to  freeze,  or  in  snow  or  ice 
which  was  beginning  to  thaw,  the  mercury  stood  at 
the  32*^'^  division  of  the  scale;  this  he  called  the  freez- 
ing point:  and  when  the  instrument  was  placed  in 
boiling  water,  the  mercury  rose  to  212,  which  he 
made  the  boiling  point;  making  a  difference  of  180 
degrees  between  these  divisions. 


100 

80 
■70 

-60 

^0 
iiO 

fo 

0 
-fO 
0 

o 


:l; 


80  Hygrometer. 

But  the  more  ordinary  way  of  constructing  ther- 
mometers, at  present,  is  to  place  them,  after  they  are 
properly  filled,  in  water  just  beginning  to  freeze, 
marking  32°  for  the  freezing  point;  and  then  after- 
wards to  immerse  them  in  boiling  water,  and  to  mark 
the  other  point  212'',  dividing  the  intermediate  dis- 
tance into  180  equal  parts,  for  the  scale  of  the  instru- 
ment, which  may  be  carried  upwards  or  downwards 
to  any  greater  extent  if  it  should  be  required. 

Small  portable  thermometers,  containing  only  a 
part  of  the  scale,  are  constructed  for  those  purposes 
where  the  variation  is  not  considerable. 


Hygrometer, 

This  is  an  instrument  which  is  used  to  ascertain 
the  different  degrees  of  humidity  or  dryness  in  the 
air.  There  are  many  inventions  for  this  purpose ;  but 
pn  instrument  sufficiently  accurate  for  common  ob- 
servation may  be  made  by  fastening  a  weight  to  a 
piece  of  twisted  catgut,  and  then  suspending  it  from 
a  nail  in  the  wall ;  the  catgut  will  contract  in  damp 
weather,  and  extend  as  the  air  becomes  dryer;  and 
the  difference  may  be  shown  by  a  scale  of  equal  parts 
fixed  on  the  wall  near  the  weight. 

An  ingenious  improvement  has  been  made  to  this 
method,  by  passing  the  catgut  round  a 
pulley  and  index  at  b,  which  points  out 
the  variation,  on  a  circle  that  is  divided 
into  equal  parts  and  fixed  on  the  wainscot 
or  wall.  Another  index  may  be  fixed  to 
the  weight  d,  which  by  the  twisting  and 
untwisting  of  the  string  according  to  the 
state  of  the  atmosphere,  will  point  out 
the  changes  upon  a  horizontal  circle  at 
E  ;  thus  a  compound  instrument  may  be 
formed,  to  correct  its  own  operation. 


81 


Mr  Gun. 


This  machine  is  made  of  brass,  and  has  two  bar- 
rels, one  within  the  other ;  the  interior  barrel  receives 
the  ball  like  a  common  gun,  and  the  exterior  is  filled 
with  compressed  air  by  a  forcing  syringe  in  the  stock 


of  the  gun.  After  the  air  is  sufficiently  compressed, 
it  is  retained  by  shutting  a  valve  that  is  placed  be- 
tween  the  syringe  and  air  cylinder.  When  the  trigger 
is  drawn,  the  fall  of  the  cock  opens  another  valve, 
which  communicates  with  the  interior  or  ball  cylinder, 
then  the  compressed  air  rushes  in  with  such  force  on 
the  back  of  the  ball,  that  it  drives  it  with  great  velo- 
city out  of  the  barrel.  As  the  ball  valve  shuts  instantly, 
a  number  of  shot  may  be  discharged  by  one  operation 
of  the  syringe ;  but  as  the  compressed  air  loses  a  part 
of  its  force  by  every  discharge,  the  velocity  of  the 
balls,  will  regularly  decrease  till  the  compression  is 
renewed. 

The  Diving  Bell. 

The  diving  bell  is  used  for  the  purpose  of  descend- 
ing to  great  depths  in  the  sea;  and  by  means  of  this 
machine  and  its  apparatus,  the  persons  who  are  con- 
tained in  it  receive  a  supply  of  air  for  their  existence, 
whilst  they  fasten  ropes  to  cannon  or  packages  which 
are  on  board  sunken  vessels,  or  to  heavy  bodies  of 
any  description  at  the  bottom  of  the  sea. 

There  are  many  forms  of  this  machine,  which  have 
different  advantages  according  to  the  different  pur- 
pgses  for  which  they  are  employed^  The  following 


82 


Diving  BelL 


was  invented  by  Halley,  who  made  the  first  improve- 
ments in  this  hazardous  machine. 

It  is  formed  like  a  bell,  and  is  about  three  feet 
wide  at  the  top,  five  feet  at 
the  bottom,  and  eight  feet 
high ;  containing  about  63 
cubic  feet,  or  eight  hogs- 
heads. Its  sides  are  loaded 
with  lead  to  make  it  sink  in 
the  water,  and  on  the  top 
of  the  bell,  c,  is  a  thick 
clear  glass,  to  give  light  to 
the  machine  when  it  is  im- 
mersed: D  is  a  stop  cock, 
by  which  the  impure  and 
rarefied  air  is  discharged: 
towards  the  middle,  e,  is  a 
seat  for  the  divers  to  rest 
upon,  and  a  broad  iron  rim, 
r,  is  suspended  by  lines 
from  the  bottom  of  the  bell 
for  the  men  to  stand  upon  as  occasion  may  require. 
When  the  divers  leave  the  bell  they  have  strong 
globular  caps,  with  flexible  tubes,  fastened  to  their 
heads,  these  caps  have  circular  glasses  in  front,  to 
give  light,  and  the  end  of  the  flexible  tube  is  kept  in 
the  bell  to  supply  the  divers  with  air,  whilst  they  fix 
tackles  to  those  bodies  that  are  to  be  raised.  The 
bell  is  supplied  with  air  during  the  whole  operation, 
by  barrels  loaded  with  lead,  which  contain  about  63 
gallons  each.  The  lower  part  of  the  barrel  is  open, 
and  the  water  compresses  the  air  in  the  upper  during 
its  descent.  A  close  and  flexible  pipe,  with  a  cock  at 
the  end,  is  fixed  to  the  top  of  the  barrel,  so  that  when 
it  has  reached  the  side  of  the  machine,  the  tube  may 
be  drawn  in  and  the  cock  turned  to  admit  the  air  into 
the  bell ;  which  is  instantly  effected  by  the  great  pres- 
sure of  the  water  upon  the  air  in  the  open  part  of  the 


Diving  Bell,  83 

barrel ;  so  that  by  a  constant  supply  of  air  barrels  the 
divers  are  enabled  to  remain  some  hours  under  water. 
The  corrupt  or  rarefied  air  in  the  bell  being  com- 
pressed by  that  which  is  fresh  and  more  dense,  rises 
to  the  top  of  the  machine,  and  by  opening  the  venting 
cock  it  is  forced  out  into  the  sea.  There  are  likewise 
signal  ropes  connected  with  the  machine,  for  moving 
it  from  one  situation  to  another ;  and  any  particular 
instructions  are  scratched  on  a  piece  of  sheet  lead 
and  sent  up  by  a  cord. 

Thus,  with  a  proper  machine  and  apparatus,  and 
vigilant  attention  from  those  who  wait  on  the  surface, 
even  the  depths  of  the  sea  cannot  hide  its  treasure 
from  the  enterprising  spirit  of  man ;  but  it  has  too 
frequently  happened  that,  by  carelessness  or  misfor- 
tune, in  this  operation,  the  lives  of  the  adventurers 
have  become  a  sacrifice  to  their  undertaking. 

It  has  been  already  shown,  that  the  space  occupied 
by  air  is  in  reciprocal  proportion  to  the  compressing 
power.  Therefore,  if  a  column  of  the  atmosphere  be 
equal  to  a  column  of  water  of  the  same  base,  34  feet 
in  height,  the  weight  of  twice  that  column  of  water 
would  compress  or  overcome  half  the  atmospherical 
column;  consequently,  if  the  body  of  air  which  is 
contained  in  the  bell  on  the  surface  of  the  water,  be 
let  down  to  the  depth  of  34  feet,  it  will  have  a  double 
weight  acting,  upon  it,  which  condenses  the  air  ^id 
suffers  the  water  to  rise  half  way  up  the  bell:  at  68 
feet  the  water  will  rise  up  two  thirds,  and  so  on  in  a 
reciprocal  proportion.  But  by  means  of  the  supplying 
barrels  the  divers  not  only  receive  fresh  air  for  their 
existence,*  but  the  compressive  power  of  the  water, 
which  acts  in  the  open  part  of  the  barrel,  drives  the 
condensed  air  with  such  force  into  the  bell,  as  over- 
balances the  compression  of  the  water,  and  drives  it 
out  of  the  machine.  Dr.  Halley  says,  that  he  has 
effected  this  to  such  a  degree,  that  he  has  been  at  the 


84  Sound. 

bottom  of  the  sea,  in  the  bell,  when  the  water  did  not 
rise  over  his  shoes. 

Sound. 

Sou  N  D  arises  from  a  tremulous  or  vibrating  motion 
in  elastic  bodies,  which  is  caused  by  a  stroke  or  col- 
lision, and  carried  to  the  ear  through  the  medium  of 
the  air.  Thus  the  production  of  sound  depends  on 
three  circumstances,  viz.  a  sonorous  body  to  give  the 
impression,  a  medium  to  convey  it,  and  the  ear  to  re- 
ceive it. 

Sonorous  bodies,  such  as  gold,  silver,  copper, 
iron,  brass  and  glass,  produce  strong  sounds,  in  pro- 
portion to  their  density  and  elasticity.  Lead,  wood, 
wax,  and  other  soft  bodies,  which  want  density  or 
elasticity  to  give  them  any  considerable  vibratory 
motion,  produce  heavy,  dull,  and  imperfect  sounds. 

Among  the  most  agreeable  variety  of  sounds,  we 
may  reckon  the  vibration  of  bells ;  the  tones  of  which 
vary  in  proportion  to  their  magnitude.  Glass,  from 
its  elastic  and  vibratory  quality,  produces  not  only 
powerful,  but  exquisitely  harmonious  sounds.  The 
strings  of  intruments,  such  as  the  harp,  violin,  or 
harpsichord,  produce  a  pleasing  variety  of  sounds: 
these  tones  are  varied  by  the  length,  thickness  and 
tension  of  the  strings  or  wires  that  compose  the  in- 
strument. The  condensation  of  air  in  the  mouth  of  a 
flute  or  the  pipes  of  organs,  &c.,  produces  pulses  of 
air  or  sound,  independently  of  the  elastic  vibration  of 
the  body. 

There  are  different  opinions  with  respect  to  the 
manner  in  which  the  vibratory  impulse  is  conveyed 
to  the  ear;  but  that  it  is  effected  by  the  medium  of 
the  air  is  experimentally  proved  by  the  different  de- 
grees of  sound  that  are  produced  in  different  densities 
of  air  under  the  receiver  of  an  airpump,  which  we 
have  already  explained. 


Sound.  85 

Though  air  is  the  most  usual  vehicle  by  which 
sound  is  conveyed  to  the  ear,  yet  it  is  not  less  power- 
fully transmitted  by  water,  or  a  continuity  of  hard  and 
sonorous  bodies,  where  the  air  can  have  no  possible 
operation.  Dr.  Franklin  imagines  that  with  his  ear 
under  water,  he  has  heard  the  collision  of  stones  in 
that  medium,  at  the  distance  of  a  mile.  The  scratch- 
ing of  a  pin  on  the  end  of  a  piece  of  hard  timber  may 
be  distinctly  heard  at  the  distance  of  15  or  20  feet, 
by  placing  the  ear  at  the  opposite  end. 

To  give  a  more  forcible  idea  of  vibratory  motion, 
and  its  effects  on  the  air  in  conveying  sound:  If  a 
string  be  pulled  tight,  and  be  afterwards  drawn  on 
one  side,  by  the  hand;  on  removing  the  power  it  will 
pass  over  to  the  opposite  side  to  nearly  the  same  dis- 
tance from  its  first  position ;  thus  it  will  continue  its 
motion  backwards  and  forwards,  gradually  decreasing 
the  distances,  till  the  vibratory  motion  ceases  and 
leaves  the  string  in  a  straight  line.  According  to  the 
first  law  of  motion,  this  vibration  would  continue 
perpetually,  if  it  were  not  impeded  in  its  progress  by 
the  density  of  the  air  and  the  cohesive  power  of  the 
string;  but  these  resistances  gradually  shorten  the 
distance  of  each  vibration,  till  at  length  the  impetus 
is  destroyed.  Now  as  particles  oi  air,  like  fluids  in 
general,  act  in  all  directions,  on  the  first  impulse  of 
the  string,  motion  is  communicated  to  all  the  sur- 
rounding particles,  as  if  the  sound  were  generated 
from  a  point;  which  is  the  cause  why  sounds  are 
equally  heard  on  all  sides  of  the  object  of  percussion, 
w'hen  the  air  is  not  acted  upon  by  any  other  impulse, 
•  The  mode  by  which  sound  is  communicated  may 
be  shown  thus:  When  the  finger  is  removed  from 
the  string  all  the  particles  of  air  that  lay  before  it  arc 
driven  forwards  and  condensed,  whatever  may  be  the 
velocity  of  the  vibratory  motion;  this  condensation 
acts  as  an  impetus  on  those  particles  that  are  still 
further  off,  and  gives  a  continued  motion  to  the 

K 


86 


The  Vibration  of  Extended  Strings. 


vibratory  impulse  till  it  reaches  the  ear.  When  the 
density  of  the  impressed  particles  is  relaxed  by  the 
return  of  the  string,  it  gives  the  same  kind  of  vibrat- 
ing motion  to  the  air  as  that  which  it  received  from 
the  string:  for  as  the  particles  at  a  distance  receive 
their  compression  and  relaxation  from  those  that  are 
before  them,  the  waves  or  pulses  of  the  air  cannot  go 
backwards  and  forwards  together,  which  would  pre- 
vent the  alternate  condensation  and  rarefaction,  but 
the  pulses  are  agitated  in  different  times,  therefore 
they  meet  one  another,  and  form  the  compression  by 
which  the  sound  is  transmitted. 


The  Vibration  of  Extended  Strings, 


B 


C 


\F. 


C/ 


.\^ 


Ac  COR  D I N  G  to  the  laws  of  pendulums,  we  find  tliat 
those  which  are  of  equal 
length  move  in  equal 
times,  although  they  pass 
through  different  arcs, 
thiit  is,  if  the  pendulum  ^{ 
A  B  and  c  D  be  equal,  the 
time  of  passing  through 
E  F  is  equal  to  that  of 
passing  through  g  h. 
Thus  the  vibration  of  the 
string  I  K  is  considered 
asadouble  pendulum,  os- 
cillating from  the  points 
K  and  I,  the  respective 
vibrations  of  which,  from 
the  greatest  to  the  least,  are  performed  in  the  same 
time :  this  is  the  reason  why  a  musical  string  has  the 
same  tone  from  the  beginning  of  the  vibration  to 
the  end. 

The  vibratory  motion  of  bodies  that  causes  the 
pulses  or  undulatory  motion  in  the  air,  which  pro- 


7/ 


K 


The  Vibration  of  Extended  Strings,  87 

duces  sound,  is  distinctly  seen  in  extended  strings, 
when  they  are  drawn  out  of  a  right  hne  and  left  free. 

Bells,  and  other  sonorous  bodies,  vibrate  in  like 
manner.  When  the  side  of  a  bell  is  struck  by  the 
clapper,  it  becomes  elliptical  from  the  elasticity  of  its 
metal,  and  has  its  greatest  diameter  from  the  point  of 
concussion  to  the  opposite  side;  this  decreases  as  its 
vibratory  motion  decreases,  till  it  again  resumes  its 
circular  form.  Although  the  motion  of  the  metal  may 
not  be  clearly  perceived,  the  effect  may  be  observed 
by  throwing  a  small  piece  of  paper  on  the  surface  of 
the  bell,  which  will  be  considerably  agitated  during 
the  vibration. 

The  power  of  vibratory  motion  and  the  transmis- 
sion of  sound  may  be  sensibly  felt  by  the  following 
easy  experiment.  Take  about  a  yard  of  riband  or 
string,  and  tie  it  in  the  middle  to  the  top  of  a  small 
bar  of  iron  or  a  common  poker;  then  twist  the  ex- 
tremities of  the  riband  round  the  fore  finger  of  each 
hand,  suspend  the  bar  and  place  a  finger  in  each  ear: 
W'hen  the  bar  is  struck  in  this  situation  by  some  other 
sonorous  body,  the  vibration  will  be  forcibly  felt,  and 
the  successive  impulses  will  be  heard  with  a  force 
and  tone  like  that  of  a  large  bell. 

It  has  been  already  stated,  that  the  pulsations  of 
sound  pass  through  the  air  in  every  direction,  as 
from  a  given  point.  To  make  this  more  clear,  the 
pulses  have  been  assimilated  to  the  small  waves,  or 
the  undulating  motion  of  water,  which  is  formed  in 
concentric  circles,  by  throwing  a  stone  into  a  standing 
pool:  but  as  this  representation  has  its  generating  cir- 
cles formed' in  a  horizontal  plane  only,  perhaps  it  may 
be  more  aptly  represented  by  any  spherical -coated 
substance  like  an  onion,  the  shells  of  which  may  re- 
present concentric  spheres  or  pulses,  issuing  and 
diverging  from  a  common  centre. 

Sound  is  found  to  possess  equal  velocity,  whether 
it  be  soft  or  loud,  sharp  or  dull.  Thus  tlic  tone  of  tlie 


88  The  Vibration  of  Extended  Strings. 

smallest  string  will  reach  the  ear  as  soon  as  that  of  the 
thickest,  the  softest  whisper  as  sOon  as  the  loudest 
voice,  or  the  report  of  the  smallest  pistol  in  the  time 
of  that  of  the  largest  cannon ;  but  the  distance  to  which 
the  pulses  are  carried,  depends  on  the  impetus  or 
force  of  concussion. 

The  decrease  of  sound  is  occasioned  by  the  want 
of  perfect  elasticity  in  the  air;  for  if  the  action  and 
reaction  of  the  ah'  were  perfect,  sound  would  continue 
to  an  infinite  extent ;  but  as  every  following  particle 
has  not  the  whole  motion  of  the  preceding,  the  further 
the  sound  passes  the  greater  is  the  impediment  it 
receives,  from  the  want  of  free  elasticity  in  the  air ; 
consequently  the  condensation  of  the  pulses  decreases 
till  the  impression  is  entirely  lost. 

According  to  the  opinions  of  Derham  and  others, 
sound  passes  through  the  distance  of  1142  feet  in  a 
second  of  time;  and  its  audibility  decreases  in  pro- 
portion to  the  squares  of  its  distance  from  the  object 
of  vibration,  or  according  to  the  extent  and  rarefac- 
tion of  the  concentric  shells  or  pulses  of  air  that  sur 
round  the  point  of  collision. 

Although  the  velocity  of  sound  is  a  little  impeded 
or  accelerated  by  currents  of  air,  the  distance  from 
the  object  that  generates  it  may  be  nearly  determined : 
for  suppose  the  report  of  a  cannon  be  heard  five  se- 
conds after  seeing  the  flash,  multiply  1142  feet,  the 
velocity  of  sound  in  a  second,  by  5,  this  will  give 
5710  feet,  or  rather  more  than  a  mile  for  the  distance 
of  the  observer  from  the  cannon:  as  the  velocity  of 
light  is  considered  as  instantaneous,  the  flash  is  taken 
for  the  first  moment  of  sound. 


Musical  Sounds. 


The  tone  of  a  sound  depends  on  the  time  that  the 
impression  dwells  on  the  ear,  or  the  time  that  the 
string  vibrates:  thus  the  longest  strings  have  the 
longest  vibrations,  and  produce  the  gravest  sound: 
on  the  contrary,  the  shortest  strings  have  the  shortest 
vibrations,  therefore  occupy  less  time,  and  have  the 
sharpest  sound.  The  tone  of  the  same  string  is  equal 
through  the  whole  of  its  vibrations,  from  the  greatest 
to  the  least,  as  we  have  already  stated.  The  time  of 
vibration  which  produces  different  tones,  depends  on 
the  length,  magnitude,  and  tension  of  the  strings. 

For  if  A  B  be  equal  a^  B. 

in  magnitude  and  ten- 
sion to   c   D,  and  their        ^^ 
lengths  be  in  the  pro-      i>^ 
portion  of  2  to  1,  the  ^* 
times  of  vibration  will 
be  in  the  same  propor- 
tion ;  tliat  is,  whilst  a  b 
makes  one  vibration,  c 
D  passes  through  two, 
or  the  vibrations  coincide 
shorter  string.   If  the  strings  e 

lengths,  and  have  the  same  tension  or  weight  at*  n  ; 
but  if  one  be  double  the  thickness  of  the  other,  the 
time  and  number  of  vibrations  in 
will  be  double  those  in  the  thicker. 

Or,  wlien  the  strings  are  of  equal  length  and  thick- 
ness, but  of  different  tension,  the  difference  of  time  or 
vibration  will  be  inversely  as  the  square  root  of  the 
weights  N  N ,  &c. ;  that  is,  if  the  weights  are  as  1  to 
4,  the  times  of  vibration  will  be  as  1  to  2,  the  sq\iare 
roots  of  1  and  4. 


at 


every  second   of  the 
F  and  G  H  be  of  equal 


the  smaller  string 


90  Speaking  Trumpet, 

In  wind  instruments,  where  the  sound  proceeds, 
from  the  elasticity  and  compression  of  the  air,  the 
vibrations  or  pulses  will  be  in  proportion  to  the  length 
and  width  of  the  tube  that  compresses  it. 

As  the  vibrations  of  strings  coincide  at  different 
intervals,  the  more  frequently  this  coincidence  hap- 
pens, the  more  agiecable  is  the  sensation  which  is 
produced  in  the  ear. 

The  vibrations  uniformly  coincide  when  the  strings 
are  of  the  same  length,  magnitude,  and  tension,  which 
produces  perfect  unison  or  concord.  The  next  greatest 
lumiber  of  coincident  vibrations  is  when  the  strings 
are  in  the  proportion  of  2  to  1;  that  is,  when  the 
shorter  string  is  half  the  length  of  the  longer,  and 
makes  two  vibrations,  whilst  the  siiorter  makes  but 
one :  these  are  called  octaves  or  eighths. 

M  the  vibrations  be  to  one  another  as  2  to  3,  the 
coincidence  will  be  at  the  third  of  the  shorter  string, 
and  in  music  it  is  called  a  fifth.  If  the  vibrations  be 
as  3  to  4,  they  produce  a  fourth,  and  so  on  through 
all  compound  vibrations,  which  form  concords  and 
discords,  accordingly  as  the  vibration  of  the  different 
strings  relate  to  one  another. 

When  two  strings  of  equal  tone  are  placed  near  to 
one  another,  on  striking  one,  the  pulse  or  undulatory 
motion  of  the  air  will  produce  a  sympathetic  sound  in 
the  other.  In  like  manner,  strings  of  different  lengths 
which  are  in  concord  \^  ith  each  other,  communicate 
a  vibration  throughout  the  whole  when  any  individual 
string  is  put  in  motion. 


Speaking  Trumpet. 

The  advantages  of  this  instrument  in  augmenting 
sound,  arise  from  the  reflection  of  the  pulses  on  the 
sides  of  the  tube  as  they  are  propagated  by  the  mouth. 
The  aerial  pulses,  which  are  thus  driven  through  the 


Echoes,  91 

tube,  not  only  augment  the  sound  by  increasing  the 
aerial  density  of  the  pulses,  but  also,  by  directing 
them  more  immediately  to  the  object;  likewise  the 
reflection  of  the  pulses  on  the  sides  of  the  trumpet 
receive  additional  force  from  the  elasticity  or  rever- 
beration of  the  metal;  or  rather,  every  point  of  per- 
cussion may  be  considered  as  a  part  from  which  fresh 
pulses  are  perpetually  generating. 

If  a  tube  be  continued  to  prevent  the  dispersion  of 
the  pulses,  sounds  may  be  carried  to  a  very  consider- 
able extent;  even  the  softest  whisper  may  be  distinctly 
heard  at  the  distance  of  15  or  20  feet.  The  whispering 
tubes  sometimes  surprise  and  amuse,  when  the  com- 
munication is  concealed,  and  the  ends  of  the  tubes 
terminate  in  the  mouth  and  ears  of  two  figures  set  at 
some  distance  apart.* 


Echoes, 

In  the  transmission  of  sound,  it  is  conceived  that 
every  point  of  impulse  serves  as  a  centre  for  genera- 
ting fresh  impulses  in  every  direction,  and  that  sound 
passes  through  equal  distances  in  equal  times.  Then, 
if  the  sum  of  the  right  lines  with  their  reflections  be 
equal  to  one  another,  the  times  will  be  equal ;  that  is, 
if  the  pulses  which  diverge  in  right  lines  from  a  given 
point,  be  variously  reflected  on  diflferent  sides,  the 
sound  will  return  in  equal  times  to  the  generating 
point,  or  to  any  other  where  the  distances  become 
equal :  this-  return  of  the  pulses  is  called  an  Echo. 

*  As  in  Peale's  museum.  Ed. 


92 


Echoes. 


If  aerial  pulses  be  propagated  from  the  point  a,  anA 
strike  various  points  of  the  curve 
CDEFG,  and  the  sums  of  the  respec- 
tive lines  taken  together  at  b,  be 
equal  to  one  another;  that  is,  ac  + 
cb=ad4-db,  and  ad+db=ae  + 
BE,  &c.  then  the  echo  or  reverbe- 
ration of  sound  will  be  heard  at  b  , 
as  a  common  point  formed  by  the 
equal  distances  or  tim€s  of  the  re- 
spective quantities  of  sound. 

Sounds  that  follow  one  another  are  not  distinctly 
heard  if  they  exceed  9  or  10  in  a  second  of  timt.  And 
as  sound  passes  through  1142  feet  in  a  second,  the 
pulses  of  sound  must  precede  each  other  by  \  of  1142, 
which  is  about  127  feet,  to  be  heard  distinctly  in  suc- 
cession. So  that  if  the  various  distances  through  which 
the  sound  is  propagated  do  not  exceed  a  b  by  127 
feet,  the  echo  will  not  be  formed  clearly  at  b.  If  the 
sums  of  the  lines  do  not  exceed  each  other  by  more 
than  127  feet,  the  sound  which  is  reflected  from  the 
different  points  of  reverberation,  will  arrive  so  near 
the  true  time,  that  the  difference  will  not  be  percep- 
tible to  the  ear. 


93 


HYDROSTATICS. 

Th  I  s  useful  and  interesting  part  of  Natural  Philo- 
sophy treats  of  the  motion,  pressure,  and  equilibrium 
of  fluids,  and  also  of  the  art  of  weighing  solids  in  them, 
to  determine  the  different  specific  gravities  or  relations 
of  bodies  to  one  another. 

By  the  word  Fluid  is  meant  a  body  compounded 
of  small  particles,  which  easily  give  way  to  an  impres- 
sing force,  varying  their  place  and  mixing  with  one 
.another  with  great  freedom  and  celerity.  The  consti- 
tuent parts,  or  the  particles  which  form  a  fluid,  are 
conceived  to  be  exceedingly  small,  smooth,  hard  and 
spherical ;  possessing  the  same  nature  and  qualities  as 
belong  to  bodies  in  general.  A  fluid  is  considered 
more  or  less  perfect,  as  the  particles  which  compose 
it  move  amongst  themselves  with  more  or  less  free- 
dom and  celerity.  Quicksilver  is  a  more  perfect  fluid 
than  water,  and  oil  is  more  fluent  than  honey. 

Fluids  are  not  perfectly  dense,  as  a  quantity  of  salt 
may  be  dissolved  in  water  without  augmenting  the 
bulk  of  the  water.  This  leads  us  to  imagine  that  the 
particles  of  water  are  spherical,  and  that  the  interstices 
which  are  formed  between  them  are  occupied  by  the 
salt,  in  the  same  manner  as  when  fine  sand  is  poured 
into  a  case  of  shot,  which  fills  up  the  vacuities  without 
augmenting  the  bulk. 

Fluids,  like  most  solid  bodies,  change  their  appear- 
ance by  the* different  modifications  of  heat;  for  a  supe- 
rior quantity  destroys  the  cohesive  force  of  the  par- 
ticles, and  forms  them  into  vapour,  and  an  inferior 
quantity  increases  the  cohesion,  and  forms  them  into 
a  solid  mass  of  ice.  Metals,  in  like  manner,  are  made 
fluid  by  an  excess  of  heat,  and  become  solid  as  it 
decreases. 

L 


94  Hydrostatic  Principles. 

This  changeable  quality,  which  belongs  to  bodies 
in  general,  leads  us  to  suppose  that  all  the  particles  of 
matter  are  constituted  alike,  and  that  the  different  ap- 
pearance of  bodies  arises  from  the  various  modifica- 
tions of  the  particles  which  compose  them :  be  this  as 
it  may,  one  common  property  is  clear;  that  all  bodies, 
whether  solid  or  fluid,  consist  of  heavy  particles,  the 
gravity  of  which  is  always  proportional  to  the  quantity 
of  matter  which  they  contain. 


Hydrostatic  Principles. 

As  the  principal  part  of  the  subject  of  fluids  de- 
pends on  certain  laws,  which  are  founded  on  reason 
and  experiment,  it  will  be  our  first  object  to  explain 
them. 

All  fluids,  except  air,  are  incompressible  in  any 
considerable  degree. 

The  Academy  del  Cimento,  from  the  following  ex- 
periment, supposed  water  to  be  totally  incompressible. 
A  globe  made  of  Gold,  which  is  less  porous  than  any 
other  metal,  was  completely  filled  with  water,  and  then 
closed  up:  it  was  afterwards  placed  under  a  great 
compressive  force,  which  pressed  the  fluid  through 
the  pores  of  the  metal,  and  formed  a  dew  all  over  its 
surface,  before  any  indent  could  be  made  in  the  ves- 
sel. Now^  as  the  surface  of  a  sphere  will  contain  a 
greater  quantity  than  the  same  surface  under  any  other 
form  whatever,  the  Academy  supposed  that  the  com- 
pressive power  which  was  applied  to  the  globe  must 
either  force  the  particles  of  the  fluid  into  closer  adhe- 
sion, or  drive  them  through  the  sides  of  the  vessel 
before  any  impression  could  be  made  on  its  surface ; 
for  although  the  latter  effect  took  place,  it  furnishes 
no  proof  of  the  incompressibility  of  water,  as  the  Flo- 
rentines had  no  method  of  determining  that  the  altera- 


Ht/drostatic  Principles.  95 

tion  of  figure  in  their  globe  of  gold  occasioned  such  a 
diminution  of  its  internal  capacity,  as  was  exactly 
equal  to  the  quantity  of  water  forced  into  its  pores; 
but  this  experiment  serves  to  show  the  great  minute- 
ness of  the  particles  of  a  fluid  in  penetrating  the  pores 
of  gold,  which  is  the  densest  of  all  metals. 

Mr.  Canton  brought  the  question  of  incompressi- 
bility  to  a  more  decisive  determination.  He  procured 
a  glass  tube,  of  about  two  feet  long,  with  a  ball  at  one 
end,  of  an  inch  and  a  quarter  in  diameter.  Having 
filled  the  ball  and  part  of  the  tube  with  mercury,  and 
brought  it  to  the  heat  of  SO''  of  Fahrenheit's  thermo- 
meter, he  marked  the  place  where  the  mercury  stood, 
and  then  raised  the  mercury  by  heat  to  the  top  of  the 
tube,  and  there  sealed  the  tube  hermetically;  then 
upon  reducing  the  mercury  to  the  same  degi'ee  of 
heat  as  before,  it  stood  in  the  tube  rh  of  an  inch 
higher  than  the  mark.  The  same  experiment  was  re- 
peated with  water  exhausted  of  air  instead  of  mercuiy , 
and  the  water  stood  in  the  tube  tVo  above  the  mark. 
Now,  since  the  weight  of  the  atmosphere  on  the  out- 
side of  the  ball,  without  any  counterbalance  from 
within,  will  compress  the  ball,  and  equally  raise  both 
the  mercury  and  water;  it  appears  that  the  w^ater  ex- 
pands ToV  of  an. inch  more  than  the  mercury,  by  re- 
moving the  weight  of  the  atmosphere.  From  this, 
and  other  experiments,  he  infers,  that  water  is  not 
only  compressible  but  elastic,  and  that  it  is  more 
capable  of  compressibility  in  winter  than  in  summer. 

All  fluids  gravitate  or  weigh  in  proportion  to  their 
quantity  of  matter^  not  only  in  the  open  air,  or  in  vacuo  ^ 
but  in  their  own  elements. 

Although  this  law  seems  so  consonant  to  reason,  it 
has  been  supposed  by  ancient  naturalists,  who  were 
ignorant  of  the  equal  and  general  pressure  of  all  fluids, 
that  the  component  parts  or  the  particles  of  the  same 
element  did  not  gravitate  or  rest  on  each  other.  So 


9G  Hydrostatic  Principles. 

that  the  \veight  of  a  vessel  of  water  balanced  in  aii\ 
would  be  entirely  lost  when  the  fluid  was  weighed  in 
its  own  element.  The  following  experiment  seems 
perfectly  to  decide  this  question. 

Take  a  common  bottle,  corked  close,  with  some 
shot  in  the  inside  to  make  it  sink,  and  fasten  it  to  the 
end  of  a  scale  beam ;  then  immerse  the  bottle  in  water, 
and  balance  the  weight  in  the  opposite  scale;  after- 
wards open  the  neck  of  the  bottle  and  let  it  fill  Math 
water,  which  will  cause  it  to  sink;  then  weigh  the 
bottle  again.  Now  it  will  be  found  that  the  weight  of 
the  water  which  is  contained  in  the  bottle  is  equal  to 
the  difference  of  the  w^eights  in  the  scale,  when  it  is 
balanced  in  air;  which  sufliciently  shows  that  the 
weight  of  the  water  is  the  same  in  both  situations. 

As  the  particles  of  fluids  possess  weight  as  a  com- 
mon property  of  bodies,  it  seems  reasonable  that  they 
should  possess  the  consequent  power  of  gravitation 
which  belongs  to  bodies  in  general.  Therefore,  sup- 
posing that  the  particles  which  compose  fluids  are 
equal,  their  gravitation  must  likewise  be  equal;  so 
that,  in  the  descent  of  fluids,  when  the  particles  are 
stopped  and  supported,  the  gravitation  being  equal, 
one  particle  will  not  have  more  propensity  than  ano- 
ther to  change  its  situation,  and  after  the  impelling 
force  has  subsided,  the  particles  will  remain  at  abso- 
lute rest. 

From  the  gravity  ofjluids  arises  their  pressure^  which 
is  always  proportional  to  the  gravity. 

For  if  the  particles  of  fluids  have  equal  magnitude 
and  weight,  the  gravity  or  pressure  must  be  propor- 
tional to  the  depth,  and  equal  in  every  horizontal  line 
of  fluid ;  consequently  the  pressure  on  the  bottom  of 
vessels  is  equal  in  every  part. 

The  pressure  of  fluids  upwards  is  equal  to  the  pres- 
sure downwards  at  any  given  depth. 


Hydrostatic  Principle.9.  97 


I 

4 
8 


For,  suppose  a  column  of  water  to  consist 
of  any  given  number  of  particles  acting  upon 
each  other  in  a  perpendicular  direction,  the 
first  particle  acts  upon  the  second  with  its  own 
weight  only;  and,  as  the  second  is  stationary, 
or  fixed  by  the  surrounding  particles,  according 
to  the  third  law  of  motion,  that  action  and  re- 
action are  equal,  it  is  evident  that  the  action 
or  gravity  in  the  first  is  repelled  in  an  equal  degree 
by  the  reaction  of  the  second;  and  in  like  manner  the 
second  acts  on  the  third,  with  its  own  gravity  added 
to  that  of  the  first,  but  still  the  reaction  increases  in 
an  equivalent  degree,  and  so  on  throughout  the  whole 
depth  of  the  fluid. 

The  particles  of  a  fluid  at  the  same  depths  press  each 
other  equally  in  all  directions. 

This  appears  to  rise  out  of  the  very  nature  of  fluids, 
for  as  the  particles  give  way  to  every  impressive  force, 
if  the  pressure  amongst  themselves  should  be  unequal 
the  fluid  could  never  be  at  rest,  which  is  contrary  t 
experience ;  therefore  we  conclude,  that  the  particles 
press  each  other  equally,  which  keeps  them  in  their 
own.  places.  This  principle  applies  to  the  whole  of  a 
fluid  as  well  as  a  part. 

For  if  four  or  five  glass  tubes  of  different  forms,  be 
immersed  in  water,  when  the  corks 
in  the  ends  are  taken  out,  the 
water  will  flow  through  the  va- 
rious windings  of  the  different 
tubes,  and  rise  in  all  of  them  to 
the  same  height  as  it  stands  in 
the  straight  tube.  Therefore  the  drops  of  fluids  must 
be  equally  pressed  in  all  directions  during  their  ascent 
through  the  various  angles  of  the  tube,  otherwise  the 
fluid  could  not  rise  to  the  same  height  in  them  all. 

From  the  mutual  pressure  and  equal  action  of  the 
particles  of  fluids^  the  surface  will  be  perfectly  smooth 
and  parallel  to  the  horizon. 


JJJ 


98 


Hydrostatic  Principles. 


If  from  any  exterior  cause  the  surflice  of  water  has 
some  parts  higher  than  the  rest,  these  will  sink  down 
by  the  natural  force  of  their  own  gravitation,  and  dif- 
fuse themselves  into  an  even  surfoce. 

Since  fluids  press  equally  every  ruay^  the  pressure  of 
each  particle  against  the  sides  of  a  vessels  ivill  be  pro- 
portional to  the  depth  of  the  particle  from  the  surface 
of  the  fluid. 

For  considering  the  first  particle  in  the  line  c  b  to 
have  no  other  weight  upon  it, 
there  is  no  other  pressure  on  the 
side  of  the  square  than  that  which 
arises  from  the  particle  itself;  but 
the  pressure  of  the  second  par- 
ticle is  increased  by  the  weight 
of  the  first,  and  the  third  by  the 
weisfht  of  the  first  and  second;  DlaMioi^kogisaSlB 
thus  the  pressure  keeps  augment- 
ing  arithmetically  to  the  end  of 
the  series.  Therefore,  considering  the  pressure  on 
the  side  as  a  series  in  arithmetical  progression,  be- 
ginning with  (0),  it  is  equal  to  half  the  pressure  on  the 
bottom:  for  as  every  particle  on  the  bottom  sustains 
11  others,  its  pressure  will  be  11x12=132;  but  the 
sum  of  the  series  for  the  pressure  on  the  side  of  the 
square  is  66,  which  is  equal  to  half  132.  So  that  the 
pressure  against  the  side  is  in  proportion  to  the  depth 
from  the  *surface,  and  the  whole  pressure  on  the  base 
of  the  square  is  equal  to  the  sum  of  the  pressures 
on  both  sides.  If  the  figure  be  made  to  represent  a 
cubical  vessel,  the  sum  of  the  pressures  against  the 
four  sides  would  be  twice  the  pressure^on  the  bottom, 
consequently  the  whole  force  of  the  fluid  would  be 
three  times  the  force  of  its  gravity. 

Thus  we  perceive  the  difference  between  fluids  and 
solids;  the  latter  act  solely  by  their  gravity;  but  fluids 
are  not  only  governed  by  gravity  but  by  pressure 


Hydrostatic  Principles. 


99 


likewise.  Solids  act  in  the  perpendicular  line  of  gra- 
vitation :  fluids  press  equally  in  every  direction.  The 
force  of  solids  is  in  proportion  to  the  quantity  of  mat- 
ter; but  the  force  of  fluids  is  in  proportion  to  the 
quantity  and  altitude. 

The  pressure  which  the  bottom  of  a  vessel  sustains 
from  the  fluid  contained  in  it^  under  every  form  of  the 
vessel^  is  equal  to  the  weight  of  a  column  of  the  fluid] 
the  base  of  which  is  equal  to  the  area  of  the  bottom, 
and  the  height  the  same  with  the  perpendicular  height 
of  the  fluid. 

From  what  has  already  been  said,  we  find  that  the 
pressure  amongst  the  particles  of  a  fluid  at  the  same 
depth  is  every  way  equal.  Then, 
if  the  base  and  height  of  the  ves- 
sel a  d,  (Fig.  2.)  be  equal  to  the 
base  and  height  of  a  f,  (Fig  1st.); 
the  pressure  on  the  base  of  the 
second  is  equal  to  the  pressure  on 
the  base  of  the  first,  although  the 
capacity  or  quantity  contained  in 
the  first  is  considerably  greater  than 
that  which  is  contained  in  the  se- 
cond. Make  i  k  l  m,  Fig.  1,  equal 
to  EGFH,  Fig.  2;  then  the  pressure 
on  the  bases  l  m  and  f  h  is  evi- 
dently equal,  and  if  i  m  be  parallel 
to  c  D,  there  will  be  the  same  pressure  upwards  on 
every  part  of  the  line  i  m;  and  the  pressure  at  i, 
where  the  column  of  water  touches  the  side  of  the 
vessel,  is  equal  to  the  pressure  of  any  other  column  in 
the  line  i  m  ;  therefore  a  column  extended  from  i  to  l 
would  be  equal  to  ne,  and  the  pressure  on  the  side 
of  the  vessel  at  i,  from  the  motion  of  fluids  in  eveiy 
direction,  has  a  reaction  equal  to  the  weight  of  the 
column  N  E ,  which  makes  the  pressure  of  i  k  on  the 
base  equal  the  whole  column  e  f.  This  principle  will 


KK  H 


100 


Hydrostatic  Principles. 


apply  to  any  other  lateral  point  in  c  e  ;  so  that  the 
whole,  taken  together,  makes  the  pressure  on  the 
bottom  c  D  equal  to  the  pressure  of  the  fluid  on  the 
bottom  of  the  cylindrical  vessel  e  f.  It  maybe  shown 
that  the  lateral  reaction  at  i  is  equal  to  the  weight  of 
the  column  n  e  ,  by  inserting  a  glass  tube  in  the  side 
of  the  vessel,  for  the  pressure  upwards  at  i  will  fill  up 
the  tube  to  the  surface  of  the  vessel. 

From  this  it  appears  that  the  pressure  on  the  bot- 
tom of  a  vessel,  of  what  form  soever,  is  not  according 
to  the  quantity  of  fluid  contained,  but  according  to 
the  perpendicular  height. 

If  c  D,  a  hogshead  full  of  water,  be  placed  on  it's 
end,  and  the  brass  tube  a  b  inserted  in 
the  top ;  on  filling  the  tube  with  water, 
the  compressive  force  on  the  sides  of  the 
vessel,  consequently  the  danger  of  burst- 
ing, would  be  as  great  as  if  the  whole 
column  were  carried  up  to  the  top  of  the 
tube.  As  the  bottom  of  vessels  supports 
a  pressure  proportional  to  the  height  of 
the  fluid,  so  the  sides  near  the  bottom 
have  the  greatest  force  acting  against 
them,  which  decreases  in  horizontal  lines 
to  the  surface. 

What  is  called  the  Hydrostatic  Bellows,  is  a  ma- 
chine well  calculated  to  show  that  the  pressure  of 
fluids  arises  more  from  the  altitude  than  from  the 
quantity  contained. 


specific  Gravity,  ^e. 


101 


This  is  formed  of  two  thick  boards 
A  and  B,  about  18  inches  long  and 
16  inches  wide;  these  are  joined  with 
strong  leather  in  the  manner  of  bel- 
lows, and  to  the  under  plank,  b,  is 
flistened  a  small  brass  tube,  c  d, 
which  communicates  with  the  interior 
of  the  machine.  When  the  experi- 
ment is  made,  a  small  quantity  of 
water  is  first  poured  in  to  keep  the 
top  and  bottom  asunder;  then  if  300 
weight  be  placed  on  a,  and  the  ma- 
chine be  filled  with  water,  till  it  stands 
about  three  feet  high  in  the  tub  cd; 
the  pressure  will  raise  up  the  weights 
to  the  extent  of  the  leather  that  joins  the  upper  and 
lower  surfaces  together.  This  extraordinary  power 
may  be  greatly  increased  by  a  forcing  piston  fixed  in 
the  tube.  A  similar  method  has  been  lately  adopted  by 
an  ingenious  mechanic,  in  forming  a  powerful  machine 
to  compress  hay,  cloths,  or  light  packages  of  any  de- 
scription, which  are  to  be  stowed  in  the  hold  of  a  vessel. 

On  the  Specific  Gravity  and  Density  of  Bodies. 

Dejiiiitions:  The  density  of  a  body  is  the  quantity 
of  matter  contained  under  a  given  bulk  or  magnitude, 
which  is  relative  as  the  quantity  of  matter  is  to  its 
magnitude:  for  the  greater  the  number  of  particles 
which  are  contained  in  a  given  portion  of  space,  the 
greater  is  the  density  of  the  body ;  and  the  fewer  the 
number  contained  in  a  like  space,  the  less  is  the  den- 
sity. 

The  specific  gravity  of  a  body  is  its  weight,  com- 
pared with  any  other  body  of  the  same  bulk  or  magni- 
tude. Thus  the  specific  gravity  of  lead  to  water  is  as 
10  to  1 ;  that  is,  a  cubic  inch  of  lead  is  as  heavy  as 
ten  cubical  inches  of  water. 

M 


102  Density  and  Specific 

The  specific  gravity  of  bodies  is  as  their  density. 
For  as  specific  gravity  is  the  weight  ojf  a  given  magni- 
tude, and  as  the  weight  of  bodies  is  according  to  the 
given  quantities  of  matter,  the  specific  gravity  is  as  the 
quantity  of  matter  contained  in  a  given  magnitude, 
or  as  the  density  of  that  magnitude. 

The  specific  gravity  of  bodies  is  inversely  as  their 
bulk  when  their  weights  are  equal.  And  as  the  specific 
gravity  of  bodies  is  as  their  density,  the  density 
of  bodies  is  likewise  inversely  as  their  bulk  when  the 
weights  are  equal.  The  specific  gravity  of  gold  to  lead 
being  as  19  to  10:  a  bar  of  gold  an  inch  square,  and 
10  inches  in  height,  will  possess  the  same  weight  as 
a  bar  of  lead  19  inches  high  and  of  the  same  base. 
The  magnitude  or  bulk  of  a  body  is  expressed  by  a 
number,  denoting  its  relation  to  some  standard  gene- 
rally used,  as  a  cubical  inch,  foot,  &c.  The  expression 
of  the  absolute  gravity  of  a  body  is  likewise  relative, 
being  determined  by  some  arbitrary  weight,  as  an 
ounce,  pound;  &c. 

Principles  and  Experiments  demonstrating  the  Den- 
sity and  specific  Gravity  of  Bodies. 

When  a  body  is  immersed  in  a  fluid,  it  loses  just 
as  much  of  its  weight  as  is  equal  to  the  weight  of  an 
equal  bulk  of  the  fluid;  but  the  weight  lost  by  the  body 
IS  gained  by  the  fluid,  which  will  be  increased  in  its 
weight  by  w^hat  the  body  has  lost. 

For,  when  a  solid  enters  a  fluid  specifically  lighter 
than  itself,  it  displaces  as  many  particles  as  are  equal 
to  its  own  magnitude,  and  these  particles  oppose  its 
descent  with  a  force  equal  to  their  pressure  upwards, 
in  a  column  of  which  the  base  is  equal  to  the  bulk  of 
the  solid :  therefore  the  weight  of  the  solid  in  the  water 
must  be  diminished,  by  the  weight  of  the  pressure  of 
the  fluid:  but  in  as  much  as  the  gravity  of  the  solid 
exceeds  the  pressure  of  the  column  of  fluid  upwards,  it 


Gravity  of  Bodies. 


103 


descends  by  the  excess,  losing  a  part  of  its  weight 
equal  to  the  repulsive  force  of  the  fluid. 

This  principle  may  be  more  clearly  understood  by 
the  following  experiment. 

Let  E  be  one  end  of  a  scale-beam,  and  b  a  bucket 
made  to  contain  a  quantity  of  fluid 
exactly  equal  to  the  magnitude  of  the 
cylinder  a,  which  is  fastened  to  the 
bucket  and  scale-beam.  After  ba- 
lancing the  scale  at  the  opposite  end, 
immerse  the  cylinder  a  in  the  vessel 
of  water  c  f;  then  the  end  with  the 
weights  will  overbalance  the  oppo- 
site end  with  the  cylinder;  but 
when  the  bucket  b  is  filled  with  a 
quantity  of  water  equal  to  the  mag- 
nitude of  the  cylinder  a,  the  balance 
will  become  equipoised.  This  evi- 
dently shows  that  the  cylinder  loses 
as  much  of  its  own  weight  as  is  equal  to  the  weight 
of  its  magnitude  of  the  fluid ;  .for  on  adding  this 
weight,  the  inequality  ceases,  and  the  balance  is  re- 
stored. 

But  the  weight  of  the  fluid  is  increased  as  the  weight 
of  the  body  is  decreased ;  for  the  action  or  pressure  on 
the  bottom  of  the  vessel  is  augmented  in  proportion  to 
the  bulk  of  the  solid;  and  as  the  gravitation  of  fluids  is 
according  to  their  heights,  the  power  of  the  fluid  is 
increased  by  the  difference  of  the  height,  before  and 
after  the  immersion  of  the  body:  for  if  the  immersion 
of  A  raise  the  water  from  d  to  c,  the  accumulated 
weight  on  the  bottom  of  the  vessel  will  be  equal  to 
the  area  of  d  c,  which  is  equal  to  the  magnitude  of 
the  body. 

If  any  body,  lighter  than  an  equal  bulk  of  the  fluid, 
be  placed  on  its  surface,  it  will  sink,  or  descend  in  it, 


_.;iF 


104  Density  and  Specific 

till  it  has  removed  or  displaced  as  much  of  the  fluid  as 
is  equal  to  the  weight  of  the  body. 

When  a  solid  which  is  specifically  lighter  than  a 
fluid  is  placed  on  its  surface,  it  sinks  till  the  pressure 
upwards  is  equal  to  the  pressure  downwards;  then  the 
respective  powers  are  in  equilibrio,  and  the  weight  of 
the  fluid  displaced  is  equal  to  the  whole  weight  of  the 
solid.  Let  a  globular  piece  of  wood,  the  specific  gra- 
vity of  which  is  less  than  that  of  the  water,  be  set  afloat 
in  a  vessel  of  water ;  then  take  the  exact  weight  of  the 
whole,  and  observe  the  point  to  which  the  water  rises 
by  the  immersion  of  the  wood.  Now,  if  the  wood  be 
taken  out,  and  the  vessel  filled  up  to  this  point,  on 
weighing  the  vessel  again  it  will  be  found  to  have  the 
same  weight  as  when  the  wood  was  immersed,  which 
shows  that  the  weight  of  the  water  which  is  displaced, 
is  equal  to  the  weight  of  the  whole  body ;  therefore,  the 
whole  solid  is  to  the  part  immersed  as  the  specific  gra- 
vity of  the  fluid  is  to  that  of  the  solid. 

All  solids  of  equal  magnitude,  though  of  different 
specific  gravities,  lose  an  equal  weight  when  they  are 
immersed  in  the  same  fluid.  For  as  the  weight  that  all 
bodies  lose  in  water,  is  according  to  the  quantity  of 
water  displaced;  the  same  bulk  will  displace  the  same 
quantity,  consequently  it  will  lose  an  equal  weight. 
Thus  a  piece  of  brass  loses  as  mu(*h  weight  in  water 
as  a  piece  of  gold  of  the  same  magnitude,  although  the 
specific  gravity  of  the  gold  is  twice  as  much'  as  that 
of  the  brass. 

Bodies  that  have  the  same  weight,  but  different 
specific  gravities,  lose  unequal  parts  of  their  weights 
when  they  are  placed  in  the  same  fluid. 

If  a  piece  of  gold  and  a  piece  of  brass  be  balanced 
in  opposite  scales,  and  afterwards  weighed  in  water, 
it  will  be  found  that  the  gold  ovtrweighs  the  brass: 
for  when  their  weights  are  equal  their  magnitudes  are 
as  their  specific  gravities;  now  as  the  specific  gravity 


Gravity  of  Bodies,  105 

of  the  gold  is  more  than  twice  that  of  the  brass,  the 
bulk  of  the  gold  is  much  less  than  the  bulk  of  the 
brass  of  the  same  weight,  therefore  the  bulk  of  the 
gold  displaces  less  water  and  loses  less  of  its  own 
weight. 

If  these  two  bodies  be  first  balanced  in  water,  and 
then  in  air,  the  brass  in  this  case  will  overweigh  the 
gold;  for  each  of  them  loses  a  part  of  their  weight 
proportionate  to  their  bulk ;  and  as  the  bulk  of  the 
brass  is  greater  than  that  of  the  gold,  it  loses  more 
weight  in  the  water;  but  this  difference  is  restored 
when  the  bodies  are  weighed  in  air,  which  causes  the 
brass  to  preponderate. 

If  a  solid  which  is  equal  in  weight  to  an  equal  bulk 
of  fluid  be  immersed  therein;  it  will  remain  indiffer- 
ently in  any  part  of  the  fluid. 

For  as  bodies  descend  by  their  own  gravity,  if  a 
column  of  water  of  equal  gravity  and  power  be  opposed 
in  the  descent  of  the  body,  the  forces  will  destroy  each 
other  or  become  equal,  so  that  the  body  which  is  im- 
mersed will  remain  suspended  in  any  part  of  the  fluid. 

If  a  body  which  is  heavier  than  an  equal  bulk  of 
the  fluid  be  immersed  in  it,  it  will  descend  by  the 
excess  of  its  gravity  above  that  of  the  fluid. 

When  a  body  is  immersed  it  loses  a  portion  of  its 
weight  from  the  resistance  of  the  medium,  and  descends 
by  the  excess;  that  is,  if  the  gravity  of  the  descending 
body  be  equal  to  3,  and  the  resistance  of  the  medium 
equal  to  2,  the  excess  or  power  of  descent  will  be  1. 

This  relative  gravity  of  solids  by  which  they  sink 
or  swim,  is  amusingly  shown  by  the  ascent  and  de- 
scent of  glass  images  in  a  jar  of  water. 

The  images  are  made  nearly  of  the  same  specific 
gravity  with  the  water,  but  rather  lighter,  with  their 
weights  a  little  varied,  to  make  them  take  different 
situations  in  the  vessel.    As  the  bodies  of  the  images 


106  Density  and  Specific 

are  hollow,  they  contain  a  quantity  of  air, 
and  when  they  are  immersed  in  the  fluid 
A  B,  the  air  comrhunicates  with  the  wa- 
ter, by  means  of  a  small  hole  in  the  heel 
of  each  image.  The  top  of  the  vessel  c 
is  covered  with  a  bladder,  which  includes 
a  quantity  of  air  in  the  upper  part  a  d  ; 
on  pressing  the  bladder,  the  elasticity  of 
the  air  presses  on  the  water,  and  causes  it  to  com- 
press the  air  in  the  bodies  of  the  images,  which  suf- 
fers a  small  portion  of  water  to  enter  the  heel,  and  in- 
crease their  gravities ;  this  causes  the  images  to  de- 
scend :  and  when  the  pressure  of  the  air  is  relaxed  on 
the  surface,  by  taking  up  the  hand,  the  air  which  is 
contained  in  the  images  forces  the  water  out  of  their 
bodies  and  they  rise  in  the  vessel:  thus  by  varying 
the  pressure  the  images  may  be  made  to  ascend  and 
descend  at  pleasure. 

It  is  by  the  pressure  of  fluids  upwards,  that  bodies 
which  are  speciflcally  lighter  than  water  rise  in  it. 

For  if  any  solid  body  lighter  than  water  be  im- 
mersed to  a  certain  depth  e,  the  pressure 
upon  the  water  underneath  the  body  is 
equal  to  the  body  d  e,  added  to  the  co- 
lumn of  water  a  i),  which  extends  from 
the  top  of  the  body  to  the  surface  of  the  B| 
fluid :  but  the  ascending  pressure  of  the 
column  of  water  c  e  is  equal  to  a  column 
of  water  a  e  ;  therefore  inasmuch  as  the 
body  is  specifically  lighter  than  the  fluid, 
so  is  the  pressure  e  c  greater  than  e  a  ,  consequently 
this  superiority  of  pressure  forces  the  body  upwards 
to  the  surface  of  the  fluid. 

By  the  reverse  of  this  principle,  bodies  specifically 
heavier  than  water  sink  to  the  bottom :  for  if  the  body 
be  of  greater  specific  gravity  than  the  water,  the  pres- 


Gravity  of  Bodies,  107 

sure  of  the  column  a  e  is  greater  than  the  power  of 
resistance  e  c;  therefore  the  body  decends. 

Bodies  which  are  Hghter  than  water  will  not  rise  in 
it,  if  the  pressure  of  the  water  underneath  the  body 
can'  be  taken  off. 

For  example,  if  a  smooth  and  even  plate 
be  fitted  on  the  end  of  the  wooden  cylin- 
der A  c,  and  placed  exactly  on  another 
plate  B,  at  the  bottom,  of  a  vessel  of  water: 
the  cylinder  and  plate  a  c,  though  specifi- 
cally lighter  than  the  water,  will  not  as- 
cend; for  the  action  or  pressure  of  the 
fluid  upwards  being  taken  away,  the  body 
is  kept  down,  not  only  by  its  own  natural  gravity 
but  by  the  weight  of  a  column  of  water  of  the  same 
base  w^ith  the  plate,  from  the  top  of  the  body  to  the 
surface  of  the  fluid. 

Bodies  specifically  heavier  than  water  may  be  made 
to  swim  on  it  when  the  water  is  kept  from  the  upper 
surface,  till  the  descent  of  the  body  meets  with  a  co- 
lumn of  fluid  the  pressure  of  which  is  equal  to  the 
specific  gravity  of  the  body. 

As  the  specific  gravity  of  water  to  brass  is  as  1  to 
9 ;  if  a  plate  of  brass  be  fitted  into  an 
open  glass  cylinder  a  c,  which  is  sus- 
pended in  a  vessel  of  water  d  e  ,  so  that 
no  part  of  the  fluid  can  get  on  the  upper 
surface  of  the  brass;  when  it  is  sunk 
nine  times  its  own  thickness,  it  will  re- 
main floating  on  the  surface  of  the  water 
in  the  cylinder ;  pressed  upwards  by  a 
force  equal  to  the  weight  of  a  column 
of  water,  the  height  of  Avhich  is  nine  times  the  thick- 
ness of  the  plate,  so  that  it  remains  supported  by  a 
resistance  equal  to  its  pressure. 


108 


To  Determine  the  Specific  Gravity  of  Bodies. 

As  specific  gravity  implies  the  relation  of  bodies  to 
one  another,  some  standard  or  given  quantity  must 
be  adopted  by  which  these  relations  may  be  determi- 
ned, and,  for  the  sake  of  experiment,  it  is  necessary 
that  this  standard  should  be  fluid:  for  this  reason,  as 
well  as  for  conveniency,  water  is  used.  It  is  likewise 
found  that  a  cubical  foot  of  distilled  wa  er  weighs  one 
thousand  ounces  avoirdupois,  which  may  be  taken 
as  a  thousand,  or  as  unity,  to  show  the  comparative 
relation,  or  the  specific  gravity  of  bodies. 

To  make  this  subject  more  clear,  it  may  be  neces- 
sary to  repeat,  what  we  have  before  stated,  that  "when 
any  body  is  immersed  in  a  fluid  it  loses  just  as  much 
of  its  weight  as  is  equal  to  the  weight  of  an  equal 
bulk  of  the  fluid ;  but  the  weight  lost  by  the  body  is 
gained  by  the  fluid,  which  will  be  increased  in  its 
weight  by  as  much  as  the  body  has  lost." 

According  to  this  principle  we  shall  have  .three  terms 
given  to  find  a  fourth;  that  is,  the  weight  of  a  body  in 
air,  the  diflference  of  its  weight  in  air  and  in  water,  and 
the  given  specific  gravity  of  the  water,  to  find  the 
comparative  relation  of  the  body. 

Suppose  a  piece  of  gold  weighs  38  grains  in  air,  but 
when  it  ip  balanced  in  water  it  weighs  only  S&,  then 
it  loses  two  grains  by  immersion,  which  is  equal  to  the 
weight  of  the  water  displaced  by  the  gold.  Now  by 
proportion,  as  the  weight  of  the  dis])Iaced  fluid  is  to 
the  weight  of  the  gold  in  air,  so  is  the  given  number, 
to  the  specific  gravity  of  the  gold;  or 

As  2  :  38  ::  1000  :  19000,  or  1  to  19. 
That  is,  if  the  specific  gravity  of  the  water  be  1000, 
or  1,  the  specific  gravity  of  gold  will  be  19000  ot  19; 
or,  in  other  terms,  gold  is  nineteen  times  heavier  than 
water. 


To  Determine,  ^c. 


109 


By  this  means,  with  the  assistance  of  the  hydrosta- 
tic balance,  the  specific  gravity  of  bodies  in  general 
may  be  determined. 

The  hydrostatic  scales 
are  of  various  construc- 
tions, according  to  the 
accuracy  which  is  re- 
quired in  the  experi- 
ment ;  but  it  will  be  suf- 
ficient for  our  purpose  to 
describe  those  that  are 
the  least  complex  yet 
sufficiently  accurate  for 
common  experiments. 
They  are  made  neai'ly 
like  common  scales,  but 

with  much  greater  nicety,  and  the  strings  to  the  scale 
at  one  end  of  the  beam  are  shortened,  so  as  to  admit 
the  water  cylinder  and  body  underneath  it.  When  the 
experiment  is  to  be  performed,  the  body  is  fastened 
by  a  horsehair  to  the  hook  under  the  shorter  scale,  and 
then  balanced  in  air ;  it  is  afterwards  weighed  in  the 
water  cylinder,  and  the  difference  of  weight  of  the  body 
in  the  two  mediums,  shows  the  difterence  of  the  spe- 
cific gravities,  which  is  equal  in  weight  to  a  bulk  of 
water  of  the  same  magnitude  as  the  body  immersed. 
The  relative  proportion  is  then  found  in  figures,  by 
stating  the  question  as  we  have  already  shown. 

By  an  improved  balance  of  this  kind  the  different 
qualities  of  gold,  or  of  any  other  metal,  may  be  ascer- 
tained with  considerable  exactness;  for,  as  all  bodies 
weigh  in  proportion  to  the  gravitating  matter  which 
they  possess  under  the  same  bulk,  and  as  the  specific 
gravity  of  fine  gold  is  greater  than  that  of  any  other 
metal,  except  platina,  it  will  possess  a  greater  weight 
under  the  same  magnitude.  In  determining  the  quality 
©f  gold  bv  the  balance,  it  is  necessary  to  fix  on  some 

N 


110        To  Determine  the  Specific  Gravity ^  ^c, 

criterion  for  its  puriti' ;  suppose  it  to  be  the  common 
standard  gold.  First  find  the  specific  gravity  of  this 
standard  by  the  preceding  method;  then,  by  taking 
the  specific  gravity  of  any  alloyed  quantity  of  gold, 
the  diffei'ence  between  them  will  show  the  quantity 
of  alloy.  Thus,  suppose  a  new  standard  guinea  weighs 
129  grains  in  air,  and  when  it  is  weighed  in  water  it 
requires  7^  grains  in  the  water  scale  to  balance  it,  its 
weight  being  only  122  -  grains  in  the  denser  medium; 
it  is  evident,  from  what  has  been  said  of  bulk  and  gra- 
vity, that  any  inferior  alloy  would  require  a  greater 
weight  in  the  water  scale  to  restore  an  equal  balance. 

If  a  suspected  guinea  should  weigh  129  grains  in 
air,  but  on  trying  it  in  water  it  requires  8 t  grains  in  the 
water  scale  to  produce  an  equilibrium,  it  shows  the 
gold  to  be  inferior  to  the  standard,  which  only  took  Vi 
grains:  thus  the  difference  of  purity  may  be  known  in 
every  kind  of  metal,  by  the  difference  of  gravity  from 
the  standard  of  that  metal. 

The  specific  gravity  of  those  bodies  which  are  lighter 
than  water  may  be  determined  in  the  following  manner. 
Weigh  the  substance  in  air,  then  fasten  it  to  the 
bottom  of  the  water  scale  with  a  stiff  wire,  and  then 
take  the  weights  out  of  the  opposite  scale,  allowing 
for  the  weight  of  the  wire;  afterwards  immerse  the 
body  in  the  water  cylinder,  adding  weights  to  the  water 
scale  till  the  balance  is  equal;  then  add  the  weights  of 
the  two  scales  together,  and  say,  as  the  sum  of  the 
weights  is  to  its  weight  in  air,  so  is  the  specific  gravity 
of  the  water  to  that  of  the  body. 

For  as  light  bodies  do  not  rise  in  water  by  reason  of 
their  own  levity,  but  from  the  superior  density  of  the 
body  in  w^hich  they  are  placed,  the  difference  of  these 
gravities  will  be  according  to  their  difference  of  weight. 

Suppose  a  piece  of  wood  weighs  in  common  air  59.5 
grains,  and  when  it  is  fastened  to  the  water  scale  and 
immersed  it  requires  16.7  grains  in  the  water  scale  to 


Hydrometer,  111 

balance  it;  then  add  59.5  to  16.7=  76.2,  and  say,  as 
76.2  :  59.5  ::  1000  :  781,  the  specific  gravity  of  the 
wood. 

To  find  the  specific  gravity  of  fluids  is  only  to  find 
their  different  degrees  of  density,  which  may  be  done 
by  fastening  a  weight  to  the  water  scale^  and  afterwards 
immersing  it  in  the  different  fluids,  noting  the  weight 
of  the  body  in  each,  and  the  difference  of  the  weights 
will  be  the  comparative  gravity  of  each. 

To  find  the  specific  gravity  of  a  fluid  in  relation  to 
water  (suppose  brandy). 

Let  a  solid  be  fastened  to  the  water  scale  and  weigh- 
ed in  air;  suppose  the  weight  to  be  1464  grains,  but 
on  weighing  it  in  water  it  loses  445  grains,  so  that 
the  balance  weight  for  this  fluid  must  be  1464 — 445 
=«1019.  Now  place  1019  grains  in  the  weight  scale, 
and  immerse  the  other  end  in  the  brandv,  and  the  bodv 
will  descend,  requiring  38.2  grains  at  the  opposite 
end  to  restore  the  equilibrium;  then  sav,  as  445  :  38.2 
::  1000  :  86,  which,  taken  from  1000,  leaves  914 
for  the  relative  gravity  of  the  spirit  to  the  water ;  so 
that  an  equal  quantity  of  the  brandy  is  about  tV  lighter 
than  water. 

Hydrometer. 

This  is  an  instrument  principally  used  by  brewers 
and  distillers  to  determine  the  strength  of  their  liquors. 

The  neck  a  b  is  a  piece  of  brass,  or  any  o  ^ 
other  metal  which  is  graduated,  to  show  the 
diflferent  depths  to  which  the  instrument  de- 
scends in  different  gravities  of  fluids,  b  is  a 
brass  bulb  to  which  the  neck  is  fastened ; 
and  c  is  a  weight  which  is  sometimes  hung 
from  the  bottom  to  keep  the  instrument  in 
an  erect  position  when  the  bulb  is  immers- 
ed in  the  fluid;  and  at  a  is  a  small  shouldc. 
to  receive  the  weights  which  are  laid  on  the 
instrument,  to  adjust  it  to  any  particular 
depth  on  the  graduated  neck. 


1 12  Hydrometer. 

Now,  as  the  resistance  of  fluids  is  according  to  their 
density,  it  is  obvious  that  the  instrument  will  sink 
deepest  in  those  fluids  that  are  the  lightest,  and  this 
variation  is  shown  by  the  scale  or  neck.  When  the 
instrument  is  immersed,  the  fluid  whkh  is  displaced 
by  it  is  equal  in  bulk  to  that  part  of  the  instrument 
which  is  covered  by  the  water,  and  in  weight  to  the 
whole  instrument.  Then,  supposing  its  weight  to  be 
4000  grains,  the  diflferent  bulks  of  fluids  containing 
the  weight  of  4000  grains  may  be  compared,  so  that 
if  a  difference  of  iV  of  an  inch  take  place  in  the  neck 
by  immersing  it  in  two  different  fluids,  it  shows  that 
the  same  weight  of  the  liquors  diflfers  in  bulk  by  the 
magnitude  of  iV  of  an  inch  of  the  stem  of  the  instru- 
ment. 

The  specific  gravity  of  fluids  may  be  found  by 
putting  an  ounce,  or  any  other  weight,  of  distilled 
water  into  a  glass  phial,  and  marking  the  height;  then 
empty  the  bottle  and  fill  it  up  to  the  same  height  ex- 
actly with  any  other  fluid,  and  weigh  them  both  in  a 
nice  balance ;  the  difference  of  these  weights  will  be 
the  difference  of  their  specific  gravities,  for  their  bulks 
are  equal. 


U3 


TABLE  OF  SPECIFIC  GRAVITIES^ 

Supposing  Rain  Water  1000. 


Refined  Gold     .    .     .  19,640 

Refined  Silver  .     .     .  11,091 

Lead 10,130 

Copper 9,000 

Iron 7,645 

Tin 7,550 

Copper  Ore       .     .     .  3,775 

Lead  Ore      ....  6,800 

Adamant,  or  Diamond  3,400 

Cornelian      .     .     .     .  2,568 

Lapis  Lazuli      .     .     .  3,054 

Lapis  Calaminaris       .  5,000 

Common  Glass      .     .  2,620 

Chalk, 2,370 

Common  Sea  Coal      .  1,272 

Ivory 1,826 

Boxwood       .     .     *     .  1,030 

Oak 925 

Elm 600 

Ash 734 

Fir 546 

Cork 240 

Wheat 757 

Oats 472 


Dry  Pease    ....  807 

Barley 658 

Crude  Mercury     .     .  13,593 

Mercury  distilled     1  14110 

5 1 1  times             3  ' 

Alum 1,714 

Nitre 1,900 

Myrrh 1,250 

Verdigris      .     .     .     .  1,714 

Opium      .....  1,365 

Bees  Wax     ....  960 

Pitch 1,190 

Honey 1,450 

Resin 1,100 

Human  Blood  .     .     .  1,126 

Distilled  Water      .     .  993 

Spring  Water   ...  999 

Sea  Water    ....  1,030 

Aquafortis    ....  1 ,300 

Oil  of  Vitriol     .     .     .  1,700 

Oil  of  Turpentine  .     .  874 
Rectified  Spirit  of  Wine      840 

Burgundy  Wine    .    .  955 


114 


HYDRAULICS. 

Hydraulics  treat  of  the  motion  of  fluids,  and 
their  application  in  forming  water  engines  of  every 
description. 

Hydrostatics  show  the  weight  or  pressure  of  fluids 
upon  solids,  or  the  particles  of  a  fluid  upon  one  ano- 
ther when  they  remain  at  rest;  and  hydraulics  treat  of 
the  power  of  fluids  when  they  are  in  motion,  and 
therefore  of  the  force  and  construction  of  engines, 
pumps,  mills,  fountains,  and  every  otiier  description  of 
hydraulical  machines. 

Although  the  motion  of  fluids  is  sufl^ciently  known 
to  make  tjfie  effects  eminently  useful,  yet  we  are  still  ig- 
norant of  the  mass,  figure,  and  number  of  the  particles 
which  are  in  motion.  There  is  little  doubt,  if  we  were 
better  acquainted  with  the  elements  of  fluidity,  so  as 
to  determine  absolute  laws  for  the  motion  of  fluids, 
that  the  advantages  which  we  already  possess  from 
their  power  might  be  greatly  extended;  but,  situated 
as  we  are,  the  mathematician  must  be  content  to  draw 
his  deductions  from  hypothesis,  and  leave  the  natu- 
ralist to  found  his  principles  on  experiment. 

The  general  effects  of  fluids  are  considered  as  pro- 
ceeding from  the  following  causes,  viz.  their  own  na- 
tural gravity  or  pressure,  the  spring  of  compressed  air, 
or  the  compression  of  bodies  on  their  surface. 

The  most  natural  motion  of  fluids  arises  from  their 
own  gravity,  which  always  causes  them  to  attain  a 
horizontal  position  when  the  course  is  left  oj^en ;  for 
a  fluid  will  rise  to  the  same     . 

height   whether   it  passes    j- j  9 -- P 

through  the  regularly  curv-     \  /I      /^      I 

ed  conduit  A  B,  or  through      ^^^     %/    ^^ 
the  various  turnings  of  c  d, 


The  Siphon. 


115 


Ciii'^^iiii^^iw 


when  the  intermediate  heights  are  below  the  level  of 
its  extremities. 

To  supply  a  reservoir  with  water  at  b,  from  a, 
the    source 
the    spring ; 
may     be    con- 
ducted between 
the  two  places 
by  pipes  laid  on 

the  surface  of  the  earth;  and,  notwithstanding  the  ob- 
struction of  the  intervening  hill,  it  will  flow  into  the 
cistern  with  a  velocity  equal  to  that  which  it  would 
have  attained  if  it  had  been  conducted  through  the 
more  direct  course  a  d  b  ;  for  the  velocity  of  fluids 
is  uniformly  as  their  height,  which  is  here  represented 
by  the  dotted  line  a  c. 

This  modern  mode  of  conveying  water  by  pipes  is 
a  great  saving  both  in  time  and  expenditure,  when  it 
is  compared  to  those  stupendous  aqueducts  Avhich 
were  constructed  by  die  Romans;  for  in  that  sera, 
either  from  their  ignorance  of  the  pressure  of  fluids, 
or  from  their  love  of  magnificence,  they  conducted 
water  across  hills  and  vallies  by  straight-lined  ducts,, 
which  were  supported  by  immense  arches  or  columns. 


The  Siphon,  or  Crane, 

Th  e  siphon  is  a  bent  pipe  or  tube,  which  is  used 
for  emptying  vessels  of  fluids,  and  sometimes  for  con- 
veying water  from  one  place  to  another,  over  hills  or 
obstacles  that  are  higher  than  the  surface  of  the  fluid. 

If  a  small  bent  tube  e  f,  the  legs 
of  which  are  of  equal  length,  be  filled 
with  water  and  turned  do^vnwards, 
with  the  ends  suspended  horizontal- 
ly, th^  fluid  will  not  run  out;  for  the 


116 


The  Siphon. 


gravitating  power  of  the  water  is 
equal  in  each  leg,  and  the  upper 
pressure  of  the  atmosphere  is  kept 
off  by  the  form  of  the  machine,  which 
causes  the  opposing  resistance  of  the 
air  that  presses  on  the  surface  of  the 
water  in  the  extremities  of  the  pipe 
to  prevent  its  descent.  But  as  the 
weight  of  a  column  of  the  atmosphere 
is  equal  to  a  column  of  water  about 
34  feet  high  of  the  same  base,  and 
to  a  column  of  mercury  29  inches 
high  in  a  medium  state  of  the  air; 
if  the  inverted  legs  of  the  siphon  ex- 
ceed these  measures  for  the  respec- 
tive fluids,  the  gravitating  weight 
will  overcome  the  resistance  of  the  at- 
mosphere, and  the  fluid  will  run  out. 

If  the  legs  are  of  unequal  length  like  those  in  the 
siphon  G  H,  and  the  shorter  leg  be  immersed  in  a  ves- 
sel of  fluid,  on  sucking  the  longer  end  with  the  mouth 
to  produce  a  vacuum,  or  by  inverting  the  tube  full  of 
water,  the  fluid  will  run  out  of  the  vessel  till  it  reaches 
the  bottom  of  the  shorter  leg ;  for  the  orifice  h  of  the 
longer  leg  is  exposed  to  the  pressure  of  the  atmo- 
sphere; and,  as  the  fluid  is  supported  in  the  shorter  leg 
by  the  surrounding  fluid  in  the  vessel,  it  is  likewise 
supported  by  the  pressure  of  the  atmosphere  which 
acts  on  the  fluid  in  the  vessel.  Now  the  atmospheri- 
cal pressures  are  equal;  but  these  pressures  are  coun- 
teracted by  unequal  columns  of  fluid  c  i  and  i  h  ; 
therefore  the  shorter  column  g  i  is  more  pressed 
against  h  i  at  the  vertex  i,  than  the  column  i  h  is 
pressed  against  i  g  ;  consequently  the  longer  column 
must  give  way  to  the  greater  pressure,  and  the  fluid 
will  run  out  of  the  orifice  h. 

The  crane  which  is  used  bv  brewers  or  distillers  for 


Fountains.  117 

emptying  hogsheads,  is  sometimes  made  with  a  cock 
and  small  pipe  at  the  end  of  the  longer  leg  to  suck  the 
air  out  of  the  tube,  and  sometimes  with  a  cock  only; 
for  when  the  cock  is  shut  before  the  shorter  end  is 
immersed  in  the  fluid,  the  air  which  is  pent  up  in  the 
crane  prevents  the  fluid  from  rising  to  the  same  height 
as  that  which  surrounds  it;  but  on  opening  the  cock 
to  emit  the  air,  the  pressure  of  the  exterior  fluid  gives 
such  velocity  to  the  interior  in  rising  to  the  general 
surface,  that  it  is  carried  beyond  it ;  and  if  the  curved 
part  of  the  siphon  be  not  too  high  above  the  liquor  in 
the  vessel,  the  interior  fluid  will  fall  over  it  and  con- 
tinue to  flow. 


Natural  and  Artificial  Fountains. 

These  are  formed  either  by  the  pressure  of  a  supe- 
rior fluid,  or  the  pressure  of  condensed  air  on  the  sur- 
face of  water. 

According  to  the  motion  of  fluids  it  has  been  already 
stated,  that  the  gravitation  of  the  upper  parts  presses 
upon  the  lower,  till  the  whole  comes  to  a  state  of  rest 
in  a  horizontal  plane. 

Thus  the  water  which  descends  from  a  reservoir 
at  A  would  acquire  such  velocity       ^ 

from  its  gravity  as  would  carry  it  ^S<^- — -^ 

up  to  its  level  at  b,  if  the  pipe  or        ]m  \ 

tube  were  continued;  but  as  the         j  1  ^ 

pipe  terminates  at  d,  it  will  issue         j  1  |||| 

at  the  adjutage  or    aperture  with        I   %  ^fl 

a    velocity  that  would  have  car-      C»-.--->^..--"^ 
ried  it  up  to  b,  and  equal  to  that  ^^^ 

which  it  has  acquired  in  falling  from  a  to  c  ;  so  that 
the  velocity  of  fountains  at  their  adjutage  is  in  pro- 
portion to  the  perpendicular  height  of  their  extremi- 
ties ;  but  the  resistance  of  the  air  at  the  lower  extre- 

O 


/ 


118  Natural  and 

mity  breaks  the  column  of  the  fluid  and  destroys  its 
force,  \\'hich,  joined  to  friction  and  other  impedi- 
ments, prevents  the  fluid  from  reaching  the  height  of 
its  source. 

In  spouting  fluids,  or  when  water  issues  from  a 
hole  in  the  bottom  or  sides  of  vessels,  it  is  found  that 
the  velocity  of  the  fluid  is  equal  to  that  which  a  body 
acquires  by  falling  perpendicularly  through  a  space 
equal  to  the  distance  between  the  surface  of  the  water 
and  the  aperture  in  the  vessel.  *  When  the  height  of 
the  fluid  is  kept  up  by  a  constant  supply,  the  velocity 
will  be  equal,  whatever  may  be  the  density  of  the  fluid; 
for  if  tlie  pressure  be  increased  by  a  denser  fluid,  the 
issuing  quantity  will  be  greater,  as  velocities  are  always 
equal  when  the  moving  forces  are  proportional  to  the 
masses  which  they  put  in  motion.  Therefore  the  quan- 
tity of  fluid  which  issues  through  the  same  hole  in  the 
same  time  is  in  proportion  to  the  celerity  of  its  motion; 
and  as  the  velocity  of  bodies  in  falling  through  a  given 
space  is  according  to  the  squares  of  the  distance  fallen 
through,  the  velocity  of  issuing  fluids  must  be  accord- 
ing to  the  square  root  of  their  height  or  pressure. 

From  the  equal  pressure  of  fluids  in  every  direction, 
the  issuing  velocity  will  always  be  the  same  at  the 
same  depth,  whether  it  proceed  from  a  hole  sideways, 
downwards,  or  upwards. 

The  greatest  distance  to  which  water  spouts  from 
different  holes  in  the  side  of  a  vessel,  is  from  that  hole 
which  is  placed  exactly  in  the  middle,  between  the 
top  and  bottom  of  the  fluid ;  and  at  the  first  and  third 
quarters  the  projected  distances  will  nearly  be  equal. 

Artificial  fountains  are  formed  by  the  compression 
of  air  on  the  s  rface  of  a  fluid. 


*  Owing  to  the  resistance  of  the  iiir,  and  other  catises  of 
obstruction,  the  velocity  of  the  spouting  fluid  will  always  be 
less  than  that  assigned  above,  from  theory.    Ed. 


Artificial  Foimtains, 


119 


If  the  vessel  b  be  partly  filled  with  water,  and  a. 
the  upper  part  of  the  vessel,  be  filled 
with  compressed  air  by  means  of  an 
injecting  syringe,  the  pressure  of  the 
air  on  the  •  surface  of  the  fluid  will 
force  it  up  the  pipe,  and  out  of  the 
adjutage,  with  a  force  proportional  to 
the  power  of  compression. , 

But  as  this  subject  has  already  been  explained  in 
Pneumatics,  w^e  will  only  describe  a  machine  which 
acts  by  the  compression  of  air  for  raising  liquors  from 
the  cellar  to  the  bar  of 
taverns,  &c.  a  is  call- 
ed the  receiving  vessel, 
which  is  made  perfectly 
air  tight,  and  sunk  about 
half  its  depth  in  the  floor 
of  the  cellar;  the  leather 
hose  D,  is  occasionally 
used  to  empty  butts  of 
liquor  e  into  the  receiv- 
ing barrel,  through  which 
it  runs  by  its  own  natural  gravity.  After  the  re- 
ceiver is  filled  to  a  proper  height  the  communication 
is  stopped  between  the  vessels,  and  the  air  is  injected 
into  the  upper  part  of  the  receiver  by  means  of  the 
forcing  piston  and  pipe  b,  which  is  placed  near  the 
bar.  This  compressive  power  on  the  surface  compels 
the  liquor  to  ascend  through  c  c,  which  is  a  leaden 
pipe  with  a  cock  that  passes  from  the  bottom  of  the 
receiver  to  some  convenient  place  where  the  liquor  is 
to  be  drawn :  When  the  velocity  of  the  fluid  decreases 
at  the  cock,  it  may  be  instantly  renewed  by  three  or 
four  strokes  with  the  handle  of  the  piston.  The  most 
common  machine  which  is  now  used  for  raising  beer, 
is  constructed  like  the  following  pump. 


120 


The  Common  Lifting  Pump. 

This  useful  and  domestic  machine  was  invented 
about  one  hundred  and  twenty  years  before  the  birth 
of  Christ;  but  it  has  been  greatly  improved,  even  since 
the  time  of  Galileo,  when  the  pressure  of  the  atmo- 
sphere became  more  perfectly  known. 

This  pump  is  formed  of  a  long 
cylinder  of  wood  or  lead,  one  end 
of  which  stands  in  the  water  at 
the  bottom  of  the  well.  It  con- 
tains two  valves,  or  hollow  pieces 
of  wood,  which  fit  close  to  the 
cylinder,  with  lids  opening  up- 
wards; the  lower  valve  c  remains 
fixed,  but  the  upper  valve  b  is 
fastened  to  the  piston  rod,  and 
moves  up  and  down  by  the  action 
of  the  handle  or  lever. 

The  mode  of  operation.  This 
description  supposes  that  the  wa- 
ter in  the  cylinder  of  the  pump 
stands  no  higher  than  the  water 
in  the  well,  and  that  the  remainder  of  the  cylinder 
is  empty,  or  rather  occupied  by  air.  Now,  when 
the  handle  of  the  pump  is  raised  up,  the  piston  b 
sinks  towards  c,  which  condenses  the  air  between 
B  and  c,  till  its  resistance  forces  open  the  valve  or 
lid;  then  the  air  escapes  into  the  upper  and  open 
part  of  the  cylinder.  As  the  piston  rises,  the  air  which 
is  contained  between  b  and  c  becomes  rarefied,  and 
the  elasticity  of  that  portion  of  air  which  is  contained 
in  the  cyliijder,  between  the  lower  valve  c  and  the 
surface  of  the  water  in  the  well,  forces  open  the  lower 
lid,  and  a  part  of  it  escapes  into  the  rarefied  space  be- 
tween B  and  c^  which  has  been  formed  by  the  rising 


Common  Lifting  Pump,  121 

of  the  piston.  Thus,  by  a  few  strokes  of  the  handle, 
if  the  wood  or  metal  of  the  cylmder  be  sufficiently 
close  to  exclude  the  air,  and  the  piston  and  valves  be 
well  fitted  to  the  sides  of  the  pipe,  the  compressive 
power  of  the  atmosphere  will  be  removed  from  the 
surface  of  that  part  of  the  fluid  which  is  contained 
within  the  cylinder,  and  the  atmospherical  pressure 
on  the  general  surface  of  the  well  will  force  it  up  the 
barrel  to  any  height  less  than  33  or  34  feet. 

Then,  supposing  the  lower  valve  to  be  placed  at  a  less 
distance  than  S^  feet  from  the  surface,  the  ascending 
water  will  force  it  open  and  get  admitted  into  the  cylin- 
der between  c  and  b  . .  When  the  piston  descends,  the 
weight  of  the  water  upon  the  lower  valve  closes  it,  and 
the  fluid,  is  forced  through  the  upper  by  the  sinking 
of  the  piston ;  so  that,  when  the  handle  is  returned,  the 
water,  which  now  rests  on  the  upper  lid,  is  carried  to- 
wards the  top  of  the  cylinder,  and  flows  out  of  the 
spout  E ;  and  the  supply  from  the  well,  by  the  com- 
pression of  the  atmosphere  upon  its  surface,  forces 
through  the  valve  c  into  the  cylinder,  as  the  upper 
piston  raises  the  water  by  the  power  of  the  handle. 

After  the  pump  has  been  worked,  if  the  barrel  and 
pistons  be  good,  the  water  will  stand  in  the  cylinder 
close  to  the  spout,  and  ready  to  flow  on  the  first  stroke 
of  the  handle. 

As  it  is  the  pressure  of  the  atmosphere  alone  that 
forces  the  water  up  the  barrel  of  the  pump,  when  the 
lower  valve  is  more  than  33  or  34  feet  from  the  sur- 
face of  the  water  in  the  well,  the  pressure  of  the  air 
cannot  raise  it  to  the  valve,  consequently  the  machine 
would  be  useless ;  but  this  is  prevented  by  sinking 
the  lower  piston  in  the  cylinder  till  it  be  actually  with- 
in the  height  of  the  pressure,  and  by  lengthening  the 
piston  rod  of  the  upper  in  proportion  to  the  depth  of 
the  lower;  this  gives  an  additional  weight  of  fluid  to 
be  lifted  each  stroke,  and  the  power  must  be  propor^ 


122  Common  Lifting  Pump,  ^ 

tionate  at  the  handle.  But  conveniency  requires  that 
this  operation  should  be  performed  by  one  person; 
therefore  to  lessen  the  weight  of  the  column  of  water 
which  extends  from  the  upper  piston-lid  to  the  mouth 
of  the  pump,  the  diameter  of  the  cylinder  must  be  de- 
creased and  made  proportional  to  the  depth  of  the  well, 
so  that  the  power  may  be  equal  to  the  operation ;  but 
the  quantity  of  water  which  is  raised  in  an  equal  time 
will  be  less  than  when  the  diameter  of  the  cylinder  is 
greater. 

The  following  table  shows  the  diameters  of  the  bar- 
rels, and  the  quantity  which  is  discharged  in  a  minute 
at  different  depths,  by  the  power  of  a  man  of  ordinary 
strength;  supposing  him  capable  of  discharging  272 
gallons  of  water  in  a  minute,  by  a  pump  30  feet  high 
and  four  inches  in  diameter;  admitting  that  the  power 
of  the  man  was  increased  five  times  by  the  length  of 
the  lever. 


123 


Heights  of  the 
Pump  above 
the    Surface 
of  the  WeU. 

1  Diameter  of  the 
1      Bore     where 

the       Piston 

works. 

Qiiantity  of  Wa- 
ter  discharg- 
ed  in   a    Mi- 
nute. 

Feet. 

10 

Inches. 

6.93 

Galls.    Pts. 

81      6 

15 

5.66 

54      4 

20 

4.90 

40      7 

25 

4.38 

32      6 

30 

4. 

27     4 

35 

3.  70 

23      3 

40 

3.46 

20      3 

45 

3.27 

18      1 

50 

3.  10 

16      3 

55 

2.95 

14      7 

60 

2.84 

13      5 

65 

2.72 

12      4 

70 

2.62 

11      5 

75 

2.53 

10      7 

80 

2.45 

10     2 

85 

2.38 

9      5 

90 

2.31 

9      1 

95 

2.25 

8      5 

100 

2.  19 

8      1 

124 


The  Forcing  Pump. 

The  forcing  pump  is  not  only  used  to  raise  water 
from  the  well  to  the  surface  of  the  earth,  but  likewise 
to  force  it  into  reservoirs  on  the  tops  of  buildings, 
from  which  pipes  are  laid  to  convey  it  to  different 
parts  as  conveniency  requires. 

This  macliine  differs  from  the  common  pump  by 
having  a  pipe  c  joined  to  the  barrel, 
through  which  the  water  passes  into 
the  air  vessel  d  f  ;  and  by  the  com- 
pression of  the  air  which  is  contained 
in  the  upper  part  h  d,  the  fluid  is 
forced  up  a  pipe,  fixed  on  g,  to  a 
considerable  height. 

The  operation.  By  moving  the 
handle  the  air  is  exhausted  out  of 
the  barrel  i  b,  and  forced  into  the  air 
vessel,  and  the  water,  follows  up  the 
cylinder  by  atmospherical  pressure, 
in  the  same  manner  as  in  the  com- 
mon pump.  But  as  the  piston  a  is 
solid,  and  the  closing  of  the  lower 
valve  prevents  the  water  from  return- 
ing into  the  well,  it  is  forced  through 
the  pipe  and  valve  c  f  into  the  air 
vessel  D  F,  and  the  valve  f  closes 
again  by  the  pressure  which  rests  upon  it,  whilst  the 
piston  ascends  to  admit  a  fresh  supply  into  the  upper 
part  of  the  cylinder.  The  upper  part  d  of  the  air  ves- 
sel is  made  perfectly  air  tight,  and  as  the  water  rises 
in  it,  it  condenses  the  air  by  pressing  it  upwards ; 
now  the  air,  by  its  elasticity,  reacts  on  the  surface  of 
the  fluid  in  proportion  to  its  density,  and  forces  it  up 
the  pipe  h  g  Avith  a  velocity  proportional  to  the  de- 


De  la  Hirers  Pump. 


125 


grce  of  compression  upon  the  surface;  and  as  the 
elasticity  of  the  air  makes  the  pressure  perpetual,  the 
pipe  produces  a  continued  stream  during  the  rising 
and  failing  of  the  piston. 

To  gain  force,  and  a  perpetual  discharge,  the  air 
vessel  is  now  used  in  the  construction  of  engines  for 
extinguishing  lire. 

The  following  is  the  construction  of  a  pump  for 
raising  water  both  by  the  ascent  and  descent  of  the 
same  piston. 


De  la  Hire's  Pump. 


Th  e  barrel  and  pipe 
the  common  forcing 
pump,  and  likewise  the 
conveyance  pipe  f, 
which  carries  the  water 
into  the  air  vessel ;  there 
is  also  another  convey- 
ance pipe  E,  which  con- 
ducts the  water  into  the 
air  vessel  that  rises  in 
the  cylinder  d  g  c,  and 
the  piston  a  k.  works  in 
a  collar  of  leather  at  a, 
which  totally  excludes 
the  air  from  the  upper 
part  of  the  cylinder. 

The  operation.  When 
the  piston  k  "^descends  it 
shuts  the  valve  /r,  and 
forces  the  water  up  the 
pipe  F  into  the  receiver 
L,  the  valvey  of  which 
immediately  closes  by 
the  pressure  of  the  fluid. 


A  B  are  the  same  a3  those  of 


126^ 


Hair  Rope  Pump* 


Then,  if  the  air  be  exhausted  out  of  the  adjoining 
barrel  d  c  c,  which  communicates  with  the  upper 
part  of  the  cylinder  a  k,  by  the  action  of  the  piston, 
and  if  the  height  d  c  should  not  exceed  33  feet,  it  is 
evident  the  water  will  rise  up  the  barrel  and  fall 
through  the  valve  g  into  the  cylinder  a  k  as  the  pis- 
ton descends ;  but  as  the  piston  ascends,  the  pressure 
of  the  water  above  it  shuts  the  valve  gy  and  forces 
open  another  at  e  in  the  air  vessel,  by  which  it  enters 
the  receiver;  then  the  return  of  the  stroke  which 
forces  the  water  up  the  pipe  f  closes  the  valve  e  and 
opens  g  again  to  admit  the  water  into  the  cylinder. 
Thus  the  ascending  and  descending  strokes  of  the 
piston  force  a  continual  supply  of  water  into  the  air 
vessel,  whence  it  is  discharged  through  a  conducting 
pipe  by  the  elasticity  and  compression  of  the  air,  as 
in  the  preceding  machine. 


Hair  Rope  Pump, 

The  three  hair  ropes  f  pass  in 
grooves  over  two  pullies  a  b,  and 
the  lines  are  kept  extended  by  a 
weight  which  is  fastened  to  the  lower 
pulley  b;  at  c  is  a  wheel  and  han- 
dle, over  which  the  line  passes  that 
joins  them  to  a  small  multiplying 
w^heel  fastened  to  the  well  beam, 
and  this  acts  on  the  uppermost  pul- 
ley. When  the  machine  is  put  in 
motion,  as  the  hair  ropes  pass 
through  the  water  in  the  well  it 
sinks  into  their  interstices,  and  by 
the  quickness  of  their  motion  it  is 
carried  up  the  ascending  ropes  in  considerable  quan- 
tities, till  it  reaches  the  upper  pulley,  when  it  ialls  into 


Archimedes^  Screw, 


127 


the  reservoir  e  .  This  method,  simple  as  it  may  appear, 
is  now  used  to  raise  water  from  a  well  90  feet  deep, 
and  by  tolerable  exertion  it  is  capable  of  drawing  up 
about  9  gallons  a  minute. 

Archimedes^  Screw  for  raising  Water. 

This  machine,  which 
is  now  seldom  used,  is  of 
very  ancient  date,  and  is 
formed  by  a  long  cylin- 
der, with  a  spiral  tube 
from  the  bottom  to  the 
top,  through  which  the 
water  rises  till  it  flows  out 
of  the  pipe  at  its  upper 
extremity  as  it  passes  the 
under  side  of  the  cylinder. 

The  principle  is  as  fbl- ; 
lows.  When  the  machine 
is  placed  in  an  oblique  di- 
rection in  the  water,  the  fluid  enters  at  a,  the  mouth 
of  the  spiral,  and  by  the  surrounding  pressure  rises 
to  c.  When  it  has  attained  this  point,  it  cannot  after- 
wards occupy  any  other  part  of  the  spiral  than  that 
which  is  on  the  under  side ;  for  it  cannot  move  from 
c  towards  d,  because  it  is  situated  higher  above  the 
horizon;  and  as  this  will  always  be  the  same  in  every 
similar  part,  it  is  evident  that  when  the  machine  is  in 
motion,  the  water,  as  it  is  raised  by  the  spiral,  will 
always  remain  on  the  uiider  side  till  it  flows  out  of 
Ae  spouf. 


^^V,    128 


Steam  Engines* 


The  great  improvement,  as  well  as  the  complexity 
of  the  smaller  parts  of  these  powerful  machines, 
would  make  a  long  description  of  the  whole  obscure 
and  uninteresting ;  it  will  therefore  be  sufficient,  for 
the  present  purpose,  to  show  the  principle  of  their 
operation  in  the  most  simple  state  of  their  construc- 
tion. 

The  beam  a  is  placed  between  two  large  standards, 
and  turns  on  its  axis  b,  with  a  piston  and  rod  fastened 
to  each  end,  which  work  in  the  cylinders  d  k  and  g  h  ; 
the  vessel  c  is  partly  filled  with  water,  which  is  kept 
boiling  by  a  fire  underneath  it,  and  this  fills  the  upper 
part  of  the  boiler  with  a  very  powerful  elastic  steam 
or  vapour ;  by  turning  the  cock  d  the  vapour  passes 
through  the  neck  of  the  vessel  and  presses  against 
the  bottom  of  the  solid  piston  r,  which  forces  it  up 
to  the  top  of  the  cylinder.  Then  the  compression 
which  raises  the  piston  f  compels  the  piston  g,  at  the 


Steam  Engines*  12& 

opposite  end  of  the  beam,  to  descend  into  the  cylin- 
dei'  G  H,  and  work  the  pump,  which  is  of  the  same 
construction  as  the  forcing  engine  that  we  have  al- 
ready described.  The  cock  d  is  shut  when  the 
piston  G  is  to  be  raised  up  to  resume  its  stroke, 
and  the  steam  in  the  cyhnder  k  d  is  instantly  con- 
densed by  letting  in  a  small  quantity  of  water  from 
the  reservoir  through  the  pipe  l,  which,  by  destroy- 
ing the  repulsive  force  of  the  vapour,  suffers  the  pis- 
ton F  to  descend  in  the  cylinder,  by  a  superior  gravi- 
tation which  is  given  to  that  end  of  the  beam  in  its 
construction. 

When  the  boilers  are  so  large  that  the  quantity  of 
steam  is  sufficient  to  make  20  or  25  strokes  in  a  mi- 
nute, and  each  of  them  7  or  8  feet  high  in  cylinders  9 
inches  in  diameter,  the  engine  will  discharge  about 
320  hogsheads  in  an  hour. 

In  the  improved  construction  of  steam  engines,  the 
operation  of  turning  the  cocks  is  performed  by  the 
machine  itself,  which  not  only  saves  the  attention 
of  one  or  two  persons,  but  likewise  performs  the  duty 
with  much  more  regularity,  and  causes  less  danger 
from  the  dreadful  effects  of  the  vapour  when  it  is  not 
properly  discharged.  To  prevent  the  bursting  of  the 
boiler  by  any  extraordinary  expansion  of  steam,  a 
valve  or  regulator  is  made  in  the  side  of  the  vessel, 
which  is  forced  open  by  the  vapour,  to  evacuate  it 
when  it  has  acquired  a  certain  force  in  the  boiler. 

The  steam  which  is  raised  by  the  ordinary  heat  of 
boiling  water  is  about  3000  times  as  rare  as  water,  and 
31  times  as  rare  as  air;  and  the  expansive  power  of 
steam  against  the  sides  of  a  globe  of  copper  four  inches 
in  diameter,  when  the  water  is  boiling  in  it,  has  been 
calculated  at  3825016s,  weight. 

Great  improvements  have  been  made  in  steam  en- 
gines^ by  Messrs.  Boulton  and  Watt,  of  Soho,  near 
Birmingham.  One  of  these  powerful  machines,  which 


\ 

130  Steam  Engines. 

was  constructed  by  them,  now  works  a  pump  18 
inches  in  diameter,  and  600  feet  high;  the  piston  makes 
10  or  12  strokes,  of  seven  feet  long,  in  a  minute,  and 
raises  a  weight  equal  to  SOOQQlbs.  fifty  feet  high  in  the 
same  time,  which  is  performed  with  a  fifth  part  of  the 
coal  that  is  usually  consumed  by  a  common  engine. 

The  present  improvements  in  steam  engines  fit  them 
for  a  variety  of  purposes  where  great  power  is  requir- 
ed; such  as  raising  water  from  mines,  blowing  large 
bellows  to  fuse  ore,  supplying  towns  with  water,  grin- 
ding corn,  &c.  Mr.  Boulton  has  lately  constructed  an 
apparatus  for  coining,  which  moves  by  an  improved 
steam  engine.  The  machinery  is  so  ingeniously  con- 
structed, that  four  boys  of  ten  or  twelve  years  of  age 
are  capable  of  striking  30000  guineas  in  an  hour, 
and  the  machine  itself  keeps  an  accurate  account  of 
the  number  which  is  struck. 


131 


ELECTRICITY. 

Electricity  is  that  power  or  property  which 
lsome  bodies  possess  of  attracting  Hght  substances 
when  they  are  excited  by  friction.  Amber,  sealing  wax, 
resin,  glass,  and  the  tourmalin  (which  is  a  red-coloured 
transparent  fossil  found  in  the  island  of  Ceylon),  are 
of  this  description. 

The  attractive  power  of  amber  was  known  some 
centuries  before  the  Christian  sera,  but  it  was  then 
considered  as  a  mere  quality  which  was  attached  to 
that  peculiar  body.  But  electricity  is  now  supposed 
to  be  a  primary  agent  of  nature,  which  is  diffused 
throughout  the  w^hole  atmosphere,  and  enters  into 
the  minutest  pores  of  bodies  in  general.  Thus,  the 
electrical  fluid,  which  had  escaped  investigation  for 
many  ages,  is  now  become  a  principal  object  of  science. 
About  the  year  1745,  we  find  this  subject  diffusing 
widely  under  the  splendid  talents  of  Watson,  Canton, 
and  Priestley,  in  London;  Franklin,  in  America;  and 
the  Abbe  Nollet,  in  France.  In  the  hands  of  these 
philosophers  electricity  has  made  more  progress  in  a 
few  years  than  it  had  gained  in  all  the  prececling  ages. 
It  was  at  this  time  that  the  mode  of  accumulating 
electrical  fluid  on  the  surface  of  glass  was  carried  to 
its  greatest  height,  by  means  of  what  is  called  the 
Leyden  Phial,  from  the  birth-place  of  the  inventor, 
who  was  a  native  of  Leyden;  but  the  greatest  disco- 
very that  vtas  ever  made  in  electricity  was  reserved  for 
Dr.  Franklin,  in  America. 

It  had  been  imagined  that  a  similarity  existed  be- 
tween lightning  and  the  electrical  fluid;  but  Franklin 
brought  this  supposition  to  the  test,  and  proved  the 
truth  of  it  by  the  simple  m.eans  of  a  boy's  kite  covered 
with  a  silk  handkerchief  instead  of  paper,  and  some 


132  Electricity, 

wire  fastened  in  the  upper  part,  which  served  to  collect 
and  conduct  the  fluid.  When  he  had  raised  this  ma- 
chine into  the  atmosphere,  he  drew  electrical  fluid 
from  the  passing  clouds,  which  descended  through 
the  flaxen  string  of  the  kite  as  a  conductor,  and  Was 
afterwards  drawn  from  an  iron  key  which  he  tied  to 
the  line  at  a  small  distance  from  his  hand.  B}'  this 
simple  means  he  proved,  that  the  fluid  which  pro- 
duces lightning  was  exactly  the  same  as  that  which  he 
obtained  from  his  electrical  machine.  This  important 
experiment  immediately  led  to  the  formation  of  con- 
ductors to  secure  buildings,  ships,  &c.  from  the  dread- 
ful effects  of  lightning. 

No  subject  in  nature  has  given  rise  to  a  greater  va- 
riet}^  of  opinions  than  the  theory  of  electricity ;  even 
in  the  present  day  the  ideas  of  philosophers  are  much 
divided  with  respect  to  the  mode  of  its  action;  but 
this  must  remain  undetermined,  till  we  are  better 
acquainted  with  its  constituent  principles;  the  dawn 
seems  to  arise,  and  a  succeeding  age  may  discover,  that 
what  was  once  considered  as  the  individual  quality 
of  a  bimple  substance,  is  the  first  agent  of  nature;  that  it 
isdiftused  from  the  sun,  as  its  infinite  source,  through- 
out the  immensity  of  our  system,  and  that  it  not  only 
produces  light  and  fire,  which  give  life  and  energy  to 
matter,  but  that  its  reacting  power  checks  the  absorb- 
ing attraction  of  the  sun,  and  gives  motion  and  bounds 
to  the  revolving  planets. 

The  present  opinions  on  electricity  are  principally 
divided  into  two  parts;  one  relating  to  what  is  called 
vitreous  and  resinous,  and  the  other  to  positive  and 
negative  electricity.  The  former  of  these  opinions  was 
first  laid  down  by  M.  du  Fay,  and  was  afterwards 
new-modelled  by  Symer;  but  it  is  now  generally  re- 
jected. This  theory  supposes  that  electrical  matter  is 
formed  by  two  distinct  fluids,  which  are  repulsive  with 
respect  to  themselves,  but  attractive  to  one  another, 


Electricity.  133 

and  the  electricities  are  called  vitreous  and  resinous, 
from  their  respective  affinities  to  glass  and  resin.  It  is 
further  supposed,  that  these  fluids  are  attracted  by  all 
bodies,  and  exist  in  intimate  union  in  their  pores,  with- 
out any  exterior  mark  of  existence,  until  the  two  fluids 
be  brought  into  action  by  a  separation  of  their  parts, 
which  is  produced  by  friction.  When  these  electrici-^ 
ties  are  collected  and  separated  by  the  attrition  of  the 
rubber  on  the  surface  of  the  cylinder  of  an  electrical 
machine,  the  vitreous  passes  over  to  the  prime  con- 
ductor, whilst  the  resinous  is  drawn  to  the  rubber.  In 
this  state  of  separation  they  exert  their  respective  quali- 
ties, so  that  by  electrifying  light  bodies  with  each  kind 
of  fluid,  those  that  possess  the  vitreous  will  repel  each 
other,  as  well  as  those  that  are  mutually  electrified 
with  the  resinous;  but  if  two  bodies  which  are  oppo^ 
sitely  electrified  be  brought  near  together,  they  will 
attract  each  other,  and  give  and  receive  at  the  same 
moment  an  equal  portion  of  their  respective  electrici- 
ties. According  to  this  theory  the  electric  spark  has  a 
double  current,  and  the  electrified  body  will  receive 
from  any  conductor  in  the  electrical  atmosphere  an 
equivalent  of  the  opposite  fluid  to  that  which  it  gives; 
so  that  if  the  finger  be  presented  to  the  prime  conductor 
of  the  machine,  whilst  the  body  inhales  the  vitreous 
stream  from  the  conductor,  it  gives  an  equal  stream  of 
the  resinous  from  the  body;  these  quantities  are  so 
exactly  alike,  that  a  light  body  may  be  suspended  by 
the  opposing  forces  between  the  end  of  the  finger  and 
the  conductor. 

The  preceding  subject  embraces  the  immediate 
outline  of  the  double  current,  or  what  is  called  vitre- 
ous and  resinous  electricity.  The  other  theory,  of 
positive  and  negative  electricity,  was  first  taken  up 
by  Watson,  but  was  afterwards  illustrated,  digested, 
and  confirmed  by  Franklin,  and  from  thence  it  is 
called  the  Franklinian  Theory.  It  supposes  that  the 

Q  ■ 


134  Electricity, 

whole  phenomena  of  electricity  depend  on  a  subtile 
and  elastic  fluid,  entirely  of  the  same  kind,  repellent 
amongst  its  own  particles,  but  attracted  by  all  bodies, 
and  universally  disseminated  throughout  their  pores. 
When  bodies  hold  their  own  natural  quantity  undis- 
turbed, they  are  said  to  be  in  a  nonelectrified  state; 
but  when  the  natural  quantity  of  fluid  in  a  bod}  is  dis- 
turbed, either  by  adding  more  to  that  which  it  natu- 
rally possesses,  or  by  taking  away  a  part  of  its  natu- 
ral quantity,  it  has  an  electrical  appearance,  or  it  is 
in  an  acting  state.  When  a  body  possesses  more  fluid 
than  its  natural  quantity,  it  is  called  plus,  or  posi- 
tive; and  when  it  contains  less  than  its  natural  quan- 
tity, it  is  called  minus,  or  negative. 

The  progress  of  this  fluid  depends  on  the  nature 
of  the  bodies  through  which  it  passes;  those  which 
give  it  the  greatest  facility  in  its  course  are  ciilled  con- 
ductors,  and  the  fluid  is  instantaneously  transmitted 
through  them  even  to  the  greatest  distances.  Those 
bodies  the  pores  of  which  will  not  admit  the  transmis- 
sion of  electrical  fluid,  are  called  electrics,  and  are 
impermeable,  so  that  there  cannot  be  an  accumulation 
on  one  side  without  a  deficiency  on  the  other;  and 
when  the  two  sides  are  joined  together  by  a  proper 
conductor,  the  superior  quantity,  or  the  positive, 
rushes  through  it  to  the  inferior  quantity,  or  negative, 
till  the  fluid  on  both  sides  of  the  body  be  in  equilibrio, 
or  in  its  natural  state.  When  an  electric  is  rubbed  by  a 
conductor,  as  the  friction  of  the  rubber  upon  the  cylin- 
der of  an  electrical  machine,  the  fluid  is  carried  from 
one  to  the  other,  and  the  rubber  will  be  electrified  nega- 
tively ;  but  as  an  insulated  cushion  only  affords  a  small 
portion  of  electric  fluid,  a  conducting  chain  is  con- 
nected with  it,  which  gives  a  constant  supply  from  the 
negative  to  the  positive  side.  Thus  it  is  conceived 
that  bodies  differently  electrified  will  readily  approach, 


Electricity,  135 

but  that  those  which  are  equally  charged  have  an 
equal  repellency. 

Having  thus  stated  the  principal  features  of  the  two 
prevailing  theories,  it  would  be  unnecessary  to  follow 
them  both;  we  have  therefore  preferred  the  latter,  not 
only  from  its  more  general  acceptation,  but  as  it 
likewise  possesses  a  simplicity  of  principle  which  ap- 
pears consonant  to  the  general  operations  of  nature. 
The  compound  quality  of  the  fluid,  double  currents, 
and  opposing  action  of  the  first  theory,  stand  unsup- 
ported by  any  other  phenomena  of  similar  principles 
in  the  operations  of  nature;  but  the  latter  has  a  strong 
coincidence  with  the  system  of  elastic  fiuidsln  general. 

Before  we  enter  into  the  experimental  proofs  of 
positive  and  negative  electricity,  it  will  be  necessary 
to  introduce  some  preparatory  knowledge  on  this 
subject. 

First,  of  the  existence  of  the  electrical  fluid. 

If  a  glass  tube,  about  an  inch  and  a  half  in  diameter 
and  three  feet  in  length,  be  rubbed  briskly  with  a  piece 
of  leather  in  a  darkened  room,  small  divergent  flames 
will  fly  off*  with  a  crackling  noise,  and  sometimes  a 
spark  of  fire  six  or  eight  inches  long  may  be  seen 
following  the  hand  upon  the  surface  of  the  tube.  If  a 
brass  ball  be  suspended  from  the  tube  by  any  conduc- 
ting body,  such  as  a  piece  of  wire,  or  thread,  the 
electric  fluid  will  descend  through  the  conductor  and 
electrify  the  ball,  which  will  give  a  spark  to  the  knuckle, 
or  electrify  any  light  body  that  is  presented  to  it.  When 
the  ball  is  suspended  from  the  tube  by  a  silken  string 
instead  of  Wire  or  thread,  the  excitation  of  the  glass 
will  produce  no  sign  of  electricity  in  the  ball;  and  if 
the  down  of  a  feather,  or  any  other  light  body,  be 
presented  to  it,  it  will  remain  unmoved;  for,  as  silk  is 
not  a  conductor,  the  fluid  cannot  pass  from  the  glass 
to  the  ball.  Thus,  with  respect  to  electricity,  all  bodies 
are  divided  into  two  kinds,  called  conductors  and  non- 


136  Electricity, 

conductors ;  though,  in  point  of  fact,  no  body  in  nature 
can  be  considered  purely  in  either  point  of  view,  so  as 
completely  to  transmit  or  retain  the  electrical  fluid. 

The  general  class  of  conductors  comprehends  metals, 
semimetals,  ores,  and  fluids,  in  their  natural  state,  ex- 
cept air  and  oils.  Green  wood  is  likewise  a  conductor, 
but  when  it  is  baked  it  becomes  a  nonconductor. 
Many  electrics  or  nonconductors,  such  as  glass,  resin, 
air,  &c.  become  conductors  by  being  heated. 

A  conductor  cannot  be  electrified  if  it  have  any  com- 
munication with  the  earth  by  means  of  immediate 
conductors,  for  then  the  excited  fluid  will  pass  through 
the  conductmg  bodies  and  be  dispersed. 

Insulated  conductors  are  conducting  bodies  sup- 
ported or  surrounded  by  an  electric  or  nonconductor, 
so  that  tlie  communication  with  the  earth  is  cut  oflT. 
A  brass  ball  and  thread  suspended  from  a  glass  tube  is 
an  insulated  conductor;  for  on  excitation  the  fluid 
passes  from  the  nonconducting  tube  through  the  thread 
to  the  ball,  where  it  is  retained  by  the  surrounding 
atmosphere  as  an  electric ;  or,  if  the  ball  be  suspended 
by  a  silken  string,  and  an  excited  tube  be  brought  to 
the  ball  and  afterwards  taken  away,  the  electrical 
fluid  which  is  communicated  will  remain  insulated  by 
the  air  and  the  nonconducting  body  of  the  silk. 

The  greatest  quantity  of  electricity  is  obtained  by 
the  friction  of  a  conducting  body  upon  the  surface  of 
an  electric.  If  the  rubber  be  afterwards  insulated,  the 
nonconducting  surface  will  remain  charged  with  the 
electric  fluid,  and  communicate  electric  sparks  to  any 
conducting  body  that  is  presented  to  it. 

If  a  conducting  body  be  insulated  and  electrified, 
the  whole  of  the  fluid  which  is  collected  will  be  car- 
ried off*  by  a  single  spark  drawn  by  a  conducting  body; 
for,  as  the  fluid  passes  with  the  greatest  facility  through 
all  parts  of  a  conductor,  the  whole  flies  off* at  the  instant 
of  communication;  but  nonconductors  that  are  charged, 


Electrical  Machine.  137 

only  part  with  that  share  of  their  fluid  which  lies  on 
the  surface  next  to  the  conductor. 

A  mutual  attraction  exists  between  electrified  and 
nonelectrified  bodies;  for,  if  a  light  substance  be 
placed  near  an  electrified  body,  it  will  fly  towards  it 
till  it  have  obtained  the  same  intensity  of  fluid,  then  it 
will  be  repelled  and  attracted  by  any  nonelectrified 
body  that  is  near  it.  If  a  nonelectrified  body  be  set 
at  a  proper  distance  from  an  electrified  body,  and  a 
feather  be  placed  between  them,  the  feather  will  be 
alternately  attracted  and  repelled  by  each ;  for  when 
it  is  electrified  it  flies  to  the  nonelectrified  body,  and 
delivers  its  electricity,  it  is  then  attracted  and  charged 
again  by  the  other;  and  thus  it  will  continue  its 
course,  backwards  and  forwards,  till  it  have  reduced 
the  surplus  of  fluid  in  that  which  is  electrified. 


Electrical  Machine. 

As  the  excitation  which  is  produced  by  the  hand 
with  a  rubber  on  a  tube  or  plate  of  glass,  is  not  only 
very  laborious,  but  inadequate  to  the  production  of 
any  material  quantity  of  electrical  fluid,  machines  have 
been  constructed  of  various  forms  for  this  purpose ; 
some  with  spherical  glass  electrics,  some  with  cylin- 
ders, and  others  by  the  revolution  of  a  circular  plate 
of  glass  between  cushions  or  rubbers  placed  near  the 
edge ;  but  as  the  cylindrical  machine  is  the  most 
common,  and  perhaps  the  most  useful,  we  have  given 
a  description  of  it  previous  to  any  further  investiga- 
tion of  electrical  fluids. 

In  the  plate,  a  represents  a  glass  cylinder  called  an 
electric,  or  nonconducting  body,  the  axis  of  which  is 
supported  by  the  two  sides  m  m,  and  these  are  fixed 
into  the  plank  k,  which  is  the  basis  of  the  machine ; 


138 


Electrical  Machine, 


c  is  a  common  winch  or  handle  by  which  the  cylin- 
der is  turned,  and  d  is  the  cushion  or  rubber,  which 
is  supported  and  insulated  by  the  glass  pillar  f.  The 
lower  end  of  f  pass- 
es into  a  socket  that 
is  acted  upon  by  the 
screw  s,  for  the  pur- 
pose of  increasing 
or  diminishing  the 
pressure  of  the  cu- 
shion on  the  surface 
of  the  cylinder,  b  is 
a  piece  of  black  silk 
which  prevents  the 
electrical  fluid  from 
flying  off*,  and  reach- 
es nearly  to  the  re- 
ceiving points  fixed  in  the  conductor  e  e  ;  for  the 
closer  the  silk  adheres  to  the  cylinder,  the  greater  will 
be  the  degree  of  excitation,  e  e  is  a  metalhc  body, 
which  is  called  the  prime  conductor;  this  is  made  in 
various  forms,  therefore  the  present  T  form  is  not  essen- 
tially necessary;  the  receiving  points  are  fastened  to  the 
side  opposite  the  cylinder,  and  the  whole  is  supported 
from  the  frame  by  the  insulating  pillar  of  glass  g,  so 
that  the  electrical  fluid  which  is  collected  on  the  prime 
conductor  cannot  disperse,  but  remains  accumulated 
for  the  purpose  of  experiments.  A  small  quadrant 
electrometer  fixes  into  a  small  hole  in  the  conductor, 
and  shows  the  increase  or  decrease  of  the  electrical 
fluid  by  the  rising  or  falling  of  the  index  upon  the 
td^Q^  of  the  quadrant.  The  chain  l  has  one  end  fas- 
tened to  the  cushion,  and  the  other  lies  upon  the  floor 
or  table,  to  serve  as  a  conductor  for  the  electrical 
fluid  in  passing  from  the  earth  to  supply  the  machine ; 
when  this  chain  is  taken  oflT,  or  unconnected  with  the 


Electrical  Machine.  139' 

earth,  the  machine  becomes  insulated,  and  it  will  re- 
tain the  electricity  that  it  has  acquired  during  its  ope- 
ration. 

Before  the  electrical  machine  is  excited  by  turning 
the  handle,  it  must  be  carefully  wiped,  or  gently  rub- 
bed, with  an  old  silk  handkerchief,  to  free  it  from  dust, 
or  any  loose  filaments  which  may  have  attached  them- 
selves to  it,  and  likewise  to  take  away  any  damp  that 
it  may  have  acquired  by  standing  in  the  room;  all  of 
which,  particularly  the  latter,  weaken  the  excitation, 
by  serving  as  conductors  to  carry  off  the  electrical  fluid. 
In  damp  weather  it  will  be  of  considerable  advantage 
to  the  power  of  the  machine,  to  place  it  in  the  gentle 
warmth  of  the  fire  for  some  little  time  before  it  is  used. 

When  the  machine  is  perfectly  dry  and  free  from 
dust,  grease  the  cylinder  by  turning  it  against  a  piece 
of  greasy  leather  until  the  glass  be  uniformly  obscured; 
then  continue  the  motion  till  the  silk  have  wiped  oflf  part 
of  the  tallow,  and  made  the  cylinder  semitransparent. 
Now  take  an  amalgam  of  zinc  and  mercury  combined 
with  a  little  tallow,  which  may  be  bought  ready  pre- 
pared, and  spread  a  little  of  this  composition  smooth 
and  even  on  a  piece  of  leather;  then  apply  it  to  the  sur- 
face of  the  cylinder,  and  turn  the  handle  till  the  friction 
become  tolerably  strong,  which  v/ill  give  it  a  great 
degree  of  excitation,  and  prepare  it  for  the  general 
purpose  of  experiment.  A  greater  degree  of  excitation 
may  be  produced  by  rubbing  the  amalgamated  leather 
against  a  clean  cylinder  and  silk,  which  will  make  the 
machine  act  powerfully  for  the  moment,  but  this  soon 
passes  ofl^"  when  the  former  mode  is  properly  pursued, 
the  machine  will  retain  its  effect  for  some  days. 


140  Positive  and 


Positive  and  Negative  Electricity. 

When  the  machine  is  put  in  motion,  the  electrical 
fluid  which  is  collecting  will  produce  a  crackling  noise, 
and  in  a  darkened  room  the  flame  will  be  seen  spread  on 
the  surface  of  the  cylinder,  but  the  course  of  the  fluid 
cannot  be  actually  determined. 

The  advocates  for  positive  and  negative  electricity 
consider  it  as  an  elastic  fluid  capable  of  condensation 
and  rarefaction,  and  that  it  is  drawn  from  the  eartli 
through  the  chain  which  is  attached  to  the  rubber,  af- 
terwards collected  on  the  cylinder  by  the  friction  of 
the  cushion  against  it,  and  thence  taken  up  by  the 
points  and  carried  to  the  prime  conductor,  which,  by 
being  insulated,  receives  more  than  its  natural  quan- 
tity, and  the  fluid  condenses  as  it  augments.  When  the 
chain  is  taken  off"  and  the  cushion  insulated,  a  very 
small  quantity  of  electricity  is  produced  from  the  fric- 
tion of  the  rubber  and  cylinder;  this  tends  to  show- 
that  the  fluid  is  received  by  the  rubber  from  the  earth ; 
therefore,  when  the  rubber  is  insulated  as  well  as  the 
prime  conductor,  the  quantity  of  fluid  which  is  pos- 
sessed by  the  former  being  less  than  that  which  is  ac- 
cumulated by  the  latter,  it  is  called  minus,  or  nega- 
tive; and  the  fluid  which  is  collected  by  the  conduc- 
tor is  plus,  or  positive.  Whilst  this  inequality  exists 
in  bodies  which  are  brought  within  each  other's  elec- 
trical atmosphere,  the  superior  power  of  the  positive 
will  attract  the  inferior  resistance  of  the  negative,  till 
the  force  of  the  electricity  become  equal ;  but  when 
bodies,  alike  positive  or  alike  negative,  are  opposed 
to  each  other,  they  resist  with  an  equal  power  and 
clasticit3^ 

The  preceding  supposition  derives  considerable 
support  from  the  following  experiment.  If  the  con- 
ductor (^(see  the  figure  of  the  machine),  with  a  small 
point  at  o,  be  placed  on  a  brass  rod  connected  with 


Negative  Electricity,  141 

the  cushion,  and  another  with  the  brass  point  p,  be 
supported  by  the  prime  conductor,  those  bodies  which 
are  electrified  by  (^  will  not  only  be  attracted  by  the 
conductor  r  ,  but  the  electrical  fluid  will  diverge  in  a 
conkal  form  from  the  point  p,  as  emitting  its  electri- 
city, whilst  a  small,  faint,  globular  fiame  will  be  seen 
on  the  point  o,  as  if  it  \vere  imbibing  the  electric 
stream. 

But  even  this  state  of  positive  and  negative  electri- 
city is  governed  by  the  quality  of  the  cylinder  and 
rubber;  for,  if  a  glass  tube  be  made  rough  by  grind- 
ing the  surface  with  emery,  and  excited  by  soft  flan- 
nel, the  electricity  will  be  negative ;  but  if  it  be  rub- 
bed by  a  dried  oil- silk  and  whiting  it  becomes  positive  y 
even  a  polished  cylinder  may  be  rendered  negative  by 
rubbing  it  with  the  hairy  side  of  a  cat's  skin.  A  cy- 
linder made  of  baked  wood,  rubbed  with  a  smooth 
rubber  of  oiled  silk,  becomes  negative ;  but  by  rub- 
bing it  with  coarse  flannel,  it  is  rendered  positive.  If 
a  cylinder  be  made  of  sulphur  or  resin,  the  electricity 
is  the  reverse  of  that  which  is  produced  by  the  smooth 
glass  cylinder  and  rubber  of  the  usual  machines ;  for 
the  rubber  in  this  case  partakes  of  the  positive,  and 
the  cylinder,  or  the  prime  conductor,  is  electrized 
with  the  negative. 

This  difference  between  the  resin  and  glass  gave 
rise  to  what  is  called  the  double  current,  or  vitreous 
and  resinous  electricity ;  but  it  is  now  generally  sup- 
posed that  the  difference  arises  more  from  the  effect  of 
the  surfaces  that  act  on  each  other,  than  from  any  pCr 
culiar  qualities  in  the  different  bodies.* 


*  If  the  body  rubb€d  be  less  affected  by  the  friction  than  the 
rubber,  the  former  will  be  electrified-  negatively,  and  the  latter 
fiositivelij ;  and  vice  versa.  Ed. 


R 


142 

Poijits. 

It  is  experimentally  found  that  points,  attached  to 
any  conducting  body,  either  receive  or  deliver  elec- 
trical fluid  more  freely  than  flat  or  round  bodies.  For 
this  reason  the  prime  conductors  of  electrical  machines 
are  always  furnished  with  points,  to  receive  the  fluid 
with  the  greater  facility  from  the  electrical  atmosphere 
of  the  cylinder. 

To  show^  the  superior  attraction  of  points.  If  tlie 
round  knob  of  a  brass  conducting  rod  be  held  near  to 
the  prime  conductor  when  the  machine  is  in  motion, 
the  electric  spark  will  be  seen  darting  towards  it ;  but 
if  a  needle,  or  fiine  pointed  conductor,  be  presented 
even  at  twice  the  distance  of  the  knob,  the  sparks 
will  instantly  cease,  and  the  fluid  will  be  silently  drawn 
off*  by  the  point;  but  when  the  point  is  withdrawn,  the 
spark  will  immediately  recommence  and  fly  towards 
the  brass  knob.  If  this  experiment  be  performed  in  a 
darkened  room,  a  small  globular  spark  appears  at  the 
end  of  the  point  when  it  is  presented,  which  shows 
that  the  needle  receives  the  fluid  from  the  conductor. 
When  the  wire  or  needle  is  fixed  towards  the  end  of 
the  prime  conductor,  on  presenting  a  brass  knob,  or 
the  finger,  the  fluid  will  pass  off"  without  any  visible 
appearance;  but  the  electric  stream  will  produce  a 
current  like  wind,  which  may  be  sensibly  felt. 

When  the  needle  is  fixed  perpendicularly  on  the 
prime  conductor,  if  crossed 
wires,  with  their  ends  all  bent 
the  same  way,  be  balanced  on 
the  point,  the  resistance  which 
the  air  gives  to  the  electric 
current  that  issues  from  the 
points,  will  drive  the  fly  round 
with  considerable  velocitv. 


Electrical  Attraction,  bV.  143 

Another  amusing  experiment,  called  the  electrical 
orrery,  is  performed  by  means  of  the  current  which 
issues  from  electrified  points. 

A  piece  of  bent  wire  is  suspended  by  a  needle  in 
the  top  of  a  glass  stand,  and  a  small  globe  of  glass  is 
fixed  in  the  centre ;  at  one  end  of  the  wire  is  another 
needle,  which  supports  a  short 
cross  wire  bent  at  each  end; 
L  is  a  pith  ball  placed  upon  it, 
which  represents  the  earth,  m 
is  a  smaller  one  at  the  end  of 
the  wire  for  the  moon,  and  s, 
the  small  glass  globe  over  the 
stand,  may  represent  the  sun. 
When  tlie  conducting  chain  is 

connected  with  the  needle  in  the  top  of  the  stand,  and 
the  machine  is  excited,  the  sun  turns  on  his  axis,  and 
the  moon  makes  her  monthly  revolution  round  the 
earth,  whilst  the  earth  is  carried  in  its  annual  orbit 
round  the  sun. 


Electrical  Attraction  and  Repulsion. 

Electricity  attracts  all  kinds  of  bodies,  but  repels 
them  after  they  are  electrized;  for  if  a  piece  of  light 
downy  feather  be  suspended  at, about  the  distance  of 
a  foot  from  an  electrified  conductor,  it  will  be  attract- 
ed or  drawn  towards  it,  and  afterwards  repelled  or 
driven  off;  for  the  electric  atmosphere  which  sur- 
rounds the  prime  conductor  attracts  the  body  till  it 
becomes  electrified  with  a  portion  of  the  same  fluid, 
after  which  the  atmosphere  of  the  conductor  and  that 
of  the  feather  repel  each  other,  and  the  feather  is 
driven  off  till  it  have  discharged  the  fluid  w^hich  it  had 
accumulated,  then  it  returns  on  the  same  principle  as 
before. 

This  attracting  and  repelling  power  is  whimsically 


144 


Electrical  Attraction^  ^c. 


^ 


illustrated  by  droll  figures  cut  out  in  paper.    The 
figures  are  laid  on  a  metal  plate  and  stand,  which 
is  placed  exactly  under  another  brass  plate 
suspended  by  a  chain  from  the  prime  con-         | 
ductor.    When  the  machine  is  excited  the 
upper  plate  is  electrified,  and  the  attracting 
atmosphere  draws  the  figures  towards  it 
but  when  the  figures  are  electrified  by  tlv 
upper  plate,  they  are  repelled  and  fly  back 
to  the  lower  uninsulated  stand  to  discharge 
their  electricity,  and  then  they  are  attract- 
ed again  as  before;  thus  the  figures  conti- 
nue jumping  backwards^  and  forwards  till 
they  have  discharged  the  fluid  from  the 
upper  plate  and  conductor. 

The  electrified  bells  is  another  pleasing  experiment, 
which  show^s  the  attraction  and  repulsion  of  the  elec- 
trical fluid. 

Four  small  bells  are  suspended  by  small  w^ires  from 
the  end  of  two  cross  rods,  and  each  arm  suspends  a 
clapper,  hung  by  a  silken  string;  the 
upper  part  of  the  stand  is  made  of 
solid  glass,  and  the  conducting  chain 
of  the  machine  communicates  with 
the  brass  knob  and  fly  on  the  top; 
tow^ai'ds  the  lower  part  of  the  stand 
is  another  bell  larger  than  the  rest, 
which  is  uninsulated,  and  forms  part 
of  a  condifctor  with  the  earth.  When 
this  machine  is  put  in  motion,  the 
fluid  passes  down  the  conducting 
wires  and  electrifies  the  oells;  these 
attract  their  respective  clappers,  which  are  afterwards 
repulsed  and  driven  off  to  the  uninsulated  bell  in  the 
centre,  which  receives  their  electricity  and  conve}s  it 
throigh  the  bottom  of  the  stand  to  the  earth.  Thus 
the  five  bells  are  kept  continually  ringing,  which  pro- 


Ley  den  Phial,  14$ 

duces  a  pleasant  peal  when  the  tones  of  the  bells  are 
properly  varied. 

Leyden  PhiaL  , 

What  is  called  the  Leyden  phial,  is  a  glass  jar 
coated  inside  and  outside  with  tin-foil,  except  about 
two  inches  on  each  side  from  the  top  of  the 
jar  downwards,  to  prevent  the  connexion 
of  the  fluid  between  the  inside  and  outside 
when  the  glass  is  charged.  The  mouth 
of  the  jar  is  covered  by  a  piece  of  wood, 
which  receives  a  thick  brass  wire;  the 
upper  end  of  the  wire  has  a  brass  knob 
fastened  to  it,  and  the  lower  end,  which 
goes  hito  the  jar,  has  a  small  wire  or  brass 
chain  fixed  to  it,  that  communicates  with  the  bottom 
and  sides,  and  serves  as  a  conductor  to  charge  the  jar 
with  electrical  fluid. 

JVhen  the  jar  is  to  be  charged,  it  may  be  held  in 
the  hand,  or  placed  upon  a  table  with  its  knob  against 
the  knob  of  the  conductor;  on  exciting  the  machine, 
the  electrical  fluid  passes  from  the  prime  conductor 
through  the  knob  and  wire  into  the  interior  of  the  jar. 
The  iiuid-,  thus  collected  and  condensed,  will  be  of  the 
same  kind  as  that  which  surrounds  the  prime  conduc- 
tor. The  exterior  part  of  the  jar  being  uninsulated, 
gives  its  natural  electricity  to  the  earth  through  the 
medium  of  the  conducting  bodies  that  connect  them, 
and  it  will  acquire  an  electricity  of  the  same  kind  as 
that  which  belongs  to  the  rubber;  thus  the  fluid  is  insu- 
lated, with  respect  to  its  connexion  from  the  opposite 
sides  of  the  jar,  by  the  unfoiled  part  at  the  top,  and 
the  increase  in  the  interior  is  in  proportion *to  the  de- 
crease on  the  exterior  side. 

When  great   force    is  required  from  the  electric 
fluid,  a  number  of  jars  of  the  above  description  are 


146  Leyden  Phial, 

placed  on  a  metal  frame  which  forms  a  communi- 
cation between  their  outside  coatings  and  the  earth, 
and  the  insides  of  the  jars  have  conducting  wires 
which  pass  to  the  prime  conductor.  In  this  manner 
imy  number  of  jars  may  be  charged  with  the  same 
facility  as  a  single  one,  and  from  the  powerful  effect 
of  the  electric  fluid,  when  it  is  thus  collected,  it  is 
called  an  electric  battery. 

The  bottle -form  is  not  absolutely  necessary  in  com- 
bining electrical  fluid;  glass  plates  were  used  for  the 
same  purpose  before  this  invention  w^as  known ;  indeed 
it  may  be  combined  by  bodies  of  every  form,  but  as 
cylindrical  jars  offer  the  largest  surface  and  greatest 
conveniency  in  the  smallest  space,  they  are  the  more 
usually  preferred. 

Dr.  Franklin  has  shown,  in  his  theory  of  the  Leyden 
phial,  that  when  one  side  of  the  jar  is  electrified  posi- 
tively, the  opposite  side  is  electrified  negatively;  for 
whatever  quantity  be  thrown  on  one  side,  that  on  the 
other  is  diminished  in  like  proportion,  so  that  the  change 
of  an  electric  jar  is  nothing  more  than  drawing  off  the 
fluid  from  one  side  and  carrying  it  to  the  other;  for 
it  is  impossible  to  charge  one  side  of  a  jar,  unless  the 
opposite  side  have  a  conductor  to  carry  off  the  fluid 
"which  it  contains:  in  like  manner  an  electrical  jar  can- 
not be  discharged  without  a  communication  between 
the  opposite  sides  to  restore  the  electrical  fluid  to  its 
natural  quantity.  To  explain  this  subject  more  particu- 
larly :  If  the  knob  of  a  coated  jar  be  held  near  thee  on- 
ductor,  on  turning  the  machine  it  will  be  seen  by  the 
index  of  the  electrometer  when  the  jar  has  received 
its  full  charge ;  then  take  a  discharging  rod  with  a  glass 
handle,  and  bring  one  of  its  knobs  to  the  knob  of  the 
jar,  and  the  other  to  the  outside  coating,  which  forms 
a  conducting  circuit  between  the  inside  and  outside 
of  the  jar,  and  the  surcharge  of  the  interior  will  fly 
through  the  conducting  rod  to  the  exterior,  till  the 


Leyden  Phial  liF 

-powers  on  each  side  are  equal.  A  person  may  convince 
himself  of  the  transmission  and  force  of  the  fluid 
through  the  wires,  by  forming  the  conductor  himself; 
for  if  he  touch  the  coated  side  of  the  jar  with  one 
finger,  and  bring  the  other  to  the  knob  of  the  jar,  he 
will  then  receive  a  strong  shock,  which  will  be  parti- 
cularly felt,  either  m  his  wrists,  elbows,  or  breast, 
according  to  the  strength  of  the  charge.  If  the  electrical 
circuit  be  made  by  any  number  of  persons  joining 
hands,  when  the  first  and  last  touch  the  knob  and  side 
of  the  jar,  or  take  hold  of  two  pieces  of  wire  which  are 
joined  to  them,  the  whole  number  will  receive  the 
shock  at  the  same  instant,  be  it  ever  so  great;  for 
the  passage  of  electricity  is  so  instantaneous,  that  in 
a  circuit  of  wire  ten  miles  in  length  the  shock  was 
felt  at  the  same  moment  that  the  jar  was  discharged. 

In  charging  a  jar,  if  it  be  held  by  the  knob,  and  the 
coated  side  presented  to  the  conductor,  the  exterior 
will  receive  the  surcharge  of  the  fluid,  and  the  interior 
will  lose  it;  that  is,  the  outside  will  be  positive,  and  the 
inside  negative,  but  the  effects  in  the  discharge  w^ill  be 
the  same. 

If  the  jar  be  placed  on  an  insulated  stand,  with  m> 
knob  near  the  prime  conductor,  the  index  will  rise  by 
the  accumulation  of  the  fluid  upon  the  conductor;  but, 
on  trial,  it  will  be  found  that  no  electricity  has  entered 
the  jar.  It  has  been  already  observed,  that  the  elec- 
trical jar  is  charged  by  taking  the  fluid  from  one  side 
and  carrying  it  to  the  other,  so  that,  in  this  case,  as 
the  outside  has  no  communication  with  the  earth,  it 
cannot  part  with  its  natural  electricity,  therefore  none 
can  be  accumulated  on  either  side  of  the  glass. 

When  the  jar  is  thus  insulated,  if  one  of  its  sides 
communicate  with  the  rubber  by  means  of  a  wire, 
and  the  other  be  connected  with  the  prime  conductor, 
the  jar  will  be  readily  charged  with  its  own  fluid;  for 
the  electricity  on  one  side  passes  through  the  wire  to 


148  Ley  den  PhiciL 

the  rubber,  and  thence  through  the  prime  conductor 
to  the  opposite  side  of  the  jar.  If  the  knob  of  the 
electric  jar  be  placed  about  half  an  inch  from  the  end 
of  the  prime  conductor,  and  a  pointed  wire  be  pre- 
sented to  the  coated  side  of  the  jar,  the  fluid  will  be 
seen  entering  the  jar  from  the  conductor,  and  passing 
from  the  outside,  vmder  the  appearance  of  a  small  star, 
upon  the  point  of  the  wire  which  is  held  towards  it ; 
but  if  a  piece  of  wire  be  fastened  round  the  jar,  with 
an  end  projecting  towards  another  conducting  wire, 
the  fluid  will  rush  from  the  side  by  this  point,  and  di- 
verge its  luminous  rays  in  a  conical  form,  taking  the 
point  of  the  wire  as  the  vertex. 

After  having  explained  the  operation  of  the  Leyden 
phial,  it  may  be  entertaining  as  well  as  useful,  to  give 
an  account  of  some  of  those  experiments  which  are 
performed  by  means  of  accumulated  fluid  in  an  elec- 
trical jar. 

A  hole  may  be  struck  through  a  card,  by  placing  it 
against  the  tin-foil  on  the  side  of  the  jar;  for  if  one 
end  of  the  conducting  rod  be  brought  against  the 
card,  and  the  other  to  the  knob,  the  fluid  will  rush 
tlirough  the  conductor,  and  pierce  the  card  as  it  joins 
the  other  fluid  on  the  exterior  surface  of  the  jar. 

To  impregnate  the  surface  of  glass  with  gold  or 
silver  leaf.  Take  two  slips  of  glass,  about  an  inch 
broad  and  three  or  four  inches  long,  and  place  a  nar- 
row slip  of  gold  leaf,  about  an  inch  longer  than  the 
glass  plates,  between  them,  so  that  each  end  of  the 
leaf  may  project  about  half  an  inch  beyond  the 
ends  of  the  glass;  then  lay  the  whole  upon  a  non- 
conducting surface,  with  a  weight  upon  it,  and  bring 
one  end  of  the  conducting  chain  or  wire  from  the 
bottom  of  the  jar  into  contact  with  that  end  of' the 
leaf  which  is  opposite  to  it,  then  bring  one  knob  of 
the  discharging  rod  to  the  knob  of  the  jar,  and  the 
other  to  the  opposite  end  of  the  leaf;  thus  the  electric 
circuit  is  completed,  and  the  leaf  will  be  melted  and 


Leyden  Phial, 


149 


driven  into  the  surface  of  the  glass  by  the  force  of  the 
electrical  fluid. 

What  is  called  the  spotted  bottle,  is  fitted  up  like 
the  Leyden  phial,  only  the  tin- foil  coating 
is  gummed  on  in  little  square  pieces  at 
some  distance  from  each  other;  so  that 
when  the  bottle  is  charged  in  the  dark, 
the  sparks  will  be  seen  flying  across  the 
spaces,  from  one  square  to  another.  If 
it  be  discharged  gently,  by  bringing  a 
pointed  wire  gradually  to  the  knob  of  the 
jar,  the  fluid  will  pleasingly  illuminate 
the  uncoated  parts,  and  make  a  crackling  noise  in 
passing  the  spaces. 

A  double  set  of  bells  will  show  the  course  of  the 
fluid  in  a  Leyden 
phial.  Let  the  bottle 
be  placed  horizontal- 
ly in  the  frame  of  an 
insulated  stand,  with 
a  knob  and  wire  in 
each  end,  communi- 
cating independently 
with  the  inside  and 
outside  of  the  jar,  and 
let  a  set  of  bells  be  suspended  from  each  wire ;  then 
place  the  knob  b  against  the  conductor,  hang  up  the 
chain  that  lies  on  the  table,  and  charge  the  jar.  Now, 
whilst  the  jar  is  receiving  the  fluid,  the  bells  on  the 
opposite  wire  a,  which  are  connected  with  the  out- 
side of  the* jar,  will  continue  ringing.  After  the  jar 
has  received  its  charge,  unhook  the  chain  at  the  end 
B,  and  let  it  lie  on  the  table;  then  touch  the  opposite 
end  A  with  the  finger,  and  those  bells  at  b  will  begin, 
and  the  others  will  cease ;  if  the  finger  be  again  pkiced 
on  B,  those  at  a  will  commence.   Thus,  by  varying 


150 


Leyden  Phial, 


the  end,  each  set  may  be  rung,  till  the  whole  of  the 
fluid  be  discharged  out  of  the  jar. 

The  following  experiment  shows  how  buildings 
may  be  set  on  fire  by  lightning,  when  combustible 
bodies  are  laid  near  metallic  rods,  plates  of  iron,  Stc. 

Take  some  powdered  resin,  rub  and  mix  it  well  in 
dry  tow  or  cotton,  and  put  it  between  two  metallic 
balls,  which  are  placed  in  a  tin  toy  resembling  a 
house;  from  these  balls  there  are  two  wire  conduc- 
tors, one  of  which  passes  to  the  outside  of  the  charged 
jar,  and  the  other  is  connected  with  one  end  of  the 
discharging  rod.  When  the  jar  is  discharged,  the 
fluid  will  instantly  force  its  passage  between  the  balls 
in  the  house  and  fire  the  tow  or  cotton,  which  blazes 
out  of  the  windows,  and  shows  some  appearance  of  a 
house  on  fire. 

The  grand  object  of  scientifical  pursuits  is,  to  add 
to  the  benefits  of  society;  then  how  much  are  we  in- 
debted to  Franklin  for  his  great  discovery  of  electrical 
conductors  !  which  avert  the  dreadful  effects  of  light- 
ning, and  convey  it  harmlessly  to  the  earth.  The  fol- 
lowing experiment  is  formed  to  show  the  effect  that 
metallic  conductors  have  on  lightning,  and  how  ne- 
cessary they  are  for  our  security. 

A  D  represents  the  gable  end  of  a  building  which 
is  made  of  wood,  with  a  square  hole  in  the  middle; 
and  G  B  c  H  is  a  small  piece  of 
wood  that  fits  loosely  into  the 
hole,  with  a  piece  of  wire  g  h 
laid  through  its  diagonal;  a  g 
and  H  D  is  the  conducting  rod, 
Avhich  is  joined  by  g  h  for  the  ]g 
sake  of  experiment ;  i  is  the  con- 
ducting chain  which  connects 
the  bottom  of  the  conductor 
with  the  bottom  of  the  jar,  to 
complete  the  circuit  with  the 


Electric  Ba  ttery .  151 

discharging  rod  k.  Now  if  die  jar  represent  a  diun- 
der  cloud  discharging  its  fluid  in  the  direction  k  a 
towards  the  top  of  the  house,  it  will  be  attracted  by 
the  rod  and  carried  by  the  metallic  conductor  a  g  h  d 
to  the  earth,  without  injuring  the  building.  But  if 
the  jar  be  charged  again,  and  the  square  piece  of  wood 
be  reversed,  placing  g  ii  in  the  direction  c  b  ;  when 
the  jar  is  discharged  an  explosion  will  ensue,  and 
the  piece  of  wood,  which  may  be  considered  as  a  part 
of  the  building,  will  be  driven  out  with  considerable 
force,  by  the  interruption  v/hich  is  given  to  the  fluid 
in  passing  through  the  conductor. 

Electric  Batterij. 

In  an  electric  battery,  or  a  combination  of  jars,  the 
accumulated  fluid  is  capable  of  performing  powerful 
experiments;  but  great  care  must  be  taken  in  using 
it,  lest  any  person  should  chance  to  get  into  the  elec- 
trical circuit,  which  would  endanger  his  life  if  the  bat- 
tery were  laige.  When  the  battery  is  used,  it  is  like- 
wise liighly  necessaiy  to  use  the  electrometer,  to  ascer- 
tain the  height  of  the  charge. 

If  a  quire  of  paper  be  suspended  by  a  string,  and  two 
ends  of  a  conducting  wire  be  brought  near  each  side 
of  it,  and  the  circuit  completed;  on  discharging  the 
battery,  the  electric  fluid  will  pierce  a  hole  through  the 
paper  without  putting  it  in  motion. 

Or,  if  a  thick  piece  of  glass  be  placed  on  an  insula- 
ted stand,  with  a  weight  laid  upon  it,  and  the  conduct- 
ing wire  of  the  machine  be  brought  into  contact  with 
the  ends  of  the  glass;  on  discharging  the  battery,  ^part 
of  the  glass  will  be  reduced  to  powder,  or,  if  the  glass 
be  of  tolerable  thickness,  it  is  sometimes  coloured  and 
shivered  in  a  curious  manner. 

When  the  coated  surface  of  the  glass  jars  in  the 
battery  contains  about  thirty  square  feet,  the  fluid  will 
melt  brass  wire  of  considerable  thickness. 


152 


METEOROLOGY. 

There  is  not  a  subject  throughout  the  sciences 
that  interests  the  general  class  of  mankind  so  much  as 
Meteorology,  and  yet,  after  all  our  researches  into 
nature,  this  interesting  knowledge  is  more  vague  and 
suppositionary,  with  respect  to  its  first  cause,  than 
any  other  whatever. 

Our  utmost  knowledge  only  extends  to  the  estab- 
lishment of  a  few  facts  which  have  been  gathered  from 
observation,  and  in  reasoning  upon  these  facts  questions 
arise  every  moment  which  we  are  unable  to  resolve. 

However,  all  kinds  of  meteorological  phenomena 
must  chiefly  depend  on  a  circulation  of  fluid,  or  the 
change  of  water  into  new  forms,  which  is  afterwards 
regenerated  or  brought  back  into  its  original  state. 

Rain 

Is  produced  by  water  which  rises  from  the  surface 
of  the  earth  in  the  form  of  a  rare,  insensible,  and  ex- 
panded vapour.  In  the  atmosphere  its  state  is  changed 
from  vapour  to  aeriform  fluid,  and  by  some  unknown 
cause  it  is  again  changed  into  mists  and  clouds,  from 
which  it  is  gathered  into  drops,  and  then  falls  to  the 
ground  to  take  its  turn  again  in  the  common  course 
of  evaporation. 

But  the  agency  in  the  formation  of  clouds  into  rain, 
and  even  of  the  vapour  into  clouds,  has  been  very  va- 
riously considered. 

Some  suppose  that  the  cold  which  constantly  occu- 
pies the  superior  regions  of  the  air  chills  or  condenses 
the  vesicles,  or  small  bubbles  into  which  vapour  is 
formed,  at  their  arrival  from  a  warmer  situation,  and 
by  bringing  them  nearer  to  each  other,  it  causes  several 


Raitu  153 

to  join  together  in  little  masses,  which  increasing  their 
qu.uid^y  of  matter  in  a  greater  degree  than  their  surface, 
they  become  too  heavy  for  the  air,  and  so  descend  in 
rain. 

But  it  seems  difficult  to  ascribe  tliis  operation  to 
cold,  as  rain  as  ofien  happens  in  warm  as  in  cold  wea- 
ther; and  another  circumstance  appears  to  destroy  ihe 
probability  of  this  supposition,  which  is  a  remarkable 
fact,  that  the  drops  of  rain  increase  in  size  as  they  de- 
scend. On  the  top  of  a  high  hill,  for  instance,  the  drops 
of  rain  may  be  quite  small,  half  way  down  its  side  they 
will  be  found  much  larger,  and  towards  the  bottom 
the  drops  will  be  very  large,  descending  in  impetuous 
rain;  which  proves  that  the  atmosphere  still  condenses 
the  vapours  where  it  is  warm  as  well  as  where  it  is 
cold.^ 

Some  bring  in  the  assistance  of  the  wind  to  produce 
this  effect,  which,  by  beating  against  a  cloud  or  congre- 
gated mass,  drives  the  vesicles  nearer  together,  by 
which  means  they  combine  and  descend  from  their 
increase  of  gravity;  so  that  when  two  winds  blow  to- 
wards the  same  point,  the  rain  will  descend  in  greater 
quantities. 

Others  attribute  this  discharge  of  the  clouds  to 
different  changes  in  the  atmosphere,  which  alter  the 
spring  or  elasticity  of  the  air;  for  as  this  elasticity 
principally  depends  on  the  dry  terrene  exhalations, 
when  these  are  weakened  the  atmosphere  is  unable 
to  support  the  weight  of  the  clouds,  and  they  fall  to- 
the  ground.  When  these  small  vesicles  are  descending, 

*  There  seems  to  be  no  difficulty  in  ascribing  the  formation 
of  clouds  and  rain  to  the  agency  of  cold,  or  abstraction  of  heat ; 
the  clouds  being  formed  in  the  upper  regions  of  the  atmosphere, 
where  the  degree  of  cold  may  be  sufficient  to  condense  the  va- 
pours, though  the  weathet  be  warm  in  the  lower  regions ;  and 
that  the  drops  of  rain  irii.rease  in  size  as  they  fall  is  a  natural 
consequence  of  cohesive  attraction.  Ed. 


154  Rain.  *  , 

they  will  still  continue  to  fall,  although  they  may  meet 
with  an  increasing  resistance  from  the  increasing 
density  of  the  air  towards  the  earth,  and  as  they  all 
tend  to  the  same  point,  the  centre  of  the  earth,  the 
farther  they  fall  together  the  more  they  will  congregate 
or  fall  into  each  other.* 

If  such  be  the  state  of  the  atmosphere  that  these 
vesicles  descend  at  a  small  distance  from  the  surface 
of  the  earth,  the  coalitions  in  their  fall  will  not  be 
numerous,  therefore  the  drops  will  be  but  small,  form- 
ing what  is  called  a  dew;  but  when  they  rise  higher 
they  descend  as  a  mist  or  fog,  and  still  higher  as  small 
rain,  so  that  when  the  drops  are  the  greatest  the  rain 
descends  from  the  greatest  height.  When  the  ascending 
vesicles  meet  with  neither  wind  nor  cold  enough  to 

o 

condense  them,  they  will  remain  for  several  days,  and 
sometimes  weeks,  before  they  disperse,  which  pro- 
duces what  is  called  a  heavy,  thick,  or  cloudy  sky. 

But  since  the  great  improvements  which  have  been 
made  in  the  science  of  electricity,  rain  has  been  con- 
sidered as  an  electrical  phenomenon.  Beccaria  supposes 
that  rain,  hail,  and  snow,  are  produced  by  the  effects 
of  a  moderate  electricity  in  the  air;  and  that  clouds, 
that  bring  rain,  are  produced  in  the  same  manner  as 
thunder  clouds,  only  by  a  more  moderate  electricity. 
That  previous  to  rain  a  quantity  of  electric  matter 
escapes  out  of  the  earth  where  there  is  a  redundancy ; 
and  that,  as  it  ascends  into  the  higher  regions  of  the  at- 
mosphere, it  collects  and  conducts  into  its  path  a  great 
quantity  of  vapours ;  and  the  same  cause  that  collects 
them  will  condense  them  more  and  more,  till  in  the 
places  of  the  nearest  intervals  they  come  almost  into 
contact,  so  as  to  form  small  drops,  which,  uniting 
with  others  as  they  fall,  come  down  in  the  form  of  rain. 

*  This  convergence  is  certainly  too  small  to  produce  any  sen- 
sible effect.  £d. 


155 


Clouds. 


Clouds  are  composed  of  a  mass  of  vesicles,  which 
may  be  seen  in  particular  situations,  and  frequently  on 
high  mountains  where  die  clouds  float  beneath  the 
observer.  These  vesicles  keep  rising  and  falling  in  the 
air  till  they  become  in  equilibrium  with  it,  then  they 
remain  in  that  state  till  they  be  again  agitated  by  a 
change  of  levity  in  that  part  of  the  atmosphere.  When 
these  vapours  approach  within  a  certain  distance  of 
each  other,  they  lose  the  latent  fire  which  they  contain, 
and  the  vapours  tend  to  unite ;  for  it  is  now  determined 
that  a  separation  of  the  latent  heat  from  the  fluid  of 
which  vapour  is  composed,  is  attended  with  a  con- 
densation of  that  vapour  in  some  degree.  In  such 
cases  it  will  first  appear  like  a  smoke,  mist,  or  fog; 
then  as  a  cloud;  and  if  this  cause  continue  to  operate, 
the  cloud  will  produce  either  rain  or  snow,  according 
to  the  degree  of  cold  in  the  air;  but  it  is  not  easy  to 
discover  why  these  clouds  remain  so  long  suspended 
without  discharging  themselves.  For  when  the  vapours 
which  are  formed  from  known  causes  get  beyond  a 
maximum  in  the  temperature  of  the  air,  the  vesicles 
are  formed  by  a  rapid  decomposition  of  superfluous 
vapours,  and  as  soon  as  this  ceases  the  vesicles  are 
dissipated,  and  the  fluid  immediately  descends  in 
drops. 

This,  and  other  circumstances,  teach  us  to  believe 
that  there  is  some  other  agent  concerned  in  the  forma- 
tion of  clouds  besides  mere  heat  and  cold ;  this  agent  is 
now  supposed  to  be  electricity,  not  only  in  the  forma- 
tion of  clouds  of  every  description,  but  in  producing 
hail,  snow,  or  rain.  This  is  most  certain,  that  the 
clouds  which  are  formed  by  atmospheric  vapours, 
whether  they  be  rendered  visible  by  electricity  or  not, 


156  Clouds. 

contain  prodigious  quantities  of  electrical  fluid,  which 
frequently  produce  the  most  disastrous  effects. 

The  most  extraordinary  instance  of  this  kind  upon 
record  happened  in  the  island  of  Java,  in  the  East 
Indies,  in  August  1772.  On  the  llth  of  that  month, 
at  midnight,  a  bright  cloud  was  observed  covering 
a  mountain  in  the  district  called  Cheribou,  and  several 
reports  like  those  of  a  gun  were  heard  at  the  same 
time.  The  people  who  dwelt  upon  the  upper  parts  of 
the  mountain  not  being  able  to  fly  fast  enough,  a  great 
part  of  the  cloud,  about  eight  or  nine  miles  in  circum- 
ference, detached  itself  under  them,  and  was  seen  at  a 
distance  rising  and  falling  like  the  waves  of  the  sea, 
and  emitting  globes  of  fire  so  luminous  that  the  night 
became  clear  as  day.  The  effects  of  it  were  astonish- 
ing; every  thing  was  destroyed  for  twenty  miles 
round,  the  houses  were  demolished,  and  plantations 
were  buried  in  the  earth,  and  2140  people  lost  their 
lives,  beside  1500  head  of  cattle,  and  a  vast  number 
of  goats,  horses,  and  other  animals. 

The  height  of  the  clouds  from  the  surface  of  the 
earth  is  not  very  considerable,  as  we  find  persons  fre- 
quently pass  through  them  as  they  ascend  the  side  of 
a  high  mountain,  on  the  top  of  which  they  see  the 
clouds  rolling  under  their  feet.  Mr.  de  Luc  particu- 
larly mentions,  that  he  saw  his  own  shadow,  and  that 
of  the  rock  on  which  he  sat,  projected  on  a  cloud 
beneath  him,  with  a  stratum  of  clouds  extending  to  a 
considerable  distance. 

Those  clouds  which  are  the  most  highly  electrified 
are  generally  the  lowest,  not  being  more  than  seven 
or  eight  hundred  yards  from  the  earth,  even  some  of 
them  are  so  low  that  they  appear  to  touch  its  surface, 
but  the  generality  of  clouds  are  suspended  at  about 
the  height  of  a  mile. 

The  shape  of  a  cloud  is  probably  owing  to  its  elec- 
tricitv,  for  at  or  near  the  time  of  thunder,  the  clouds 


Clouds.  157 

vary  their  shape  continually,  assuming  grotesque  and 
fanciful  appearances  ;  we  may  often  perceive  the  edge 
we  are  looking  at  dissipated  in  the  place  where  it  was 
first  observed;  sometimes  the  edge  stretches  itself 
out,  whilst  the  cloud  vanishes  away.  It  frequently 
happens  that,  when  one  edge  disappears,  others  are 
formed  by  which  the  cloud  is  enlarged;  at  other  times 
the  edges  evaporate  till  the  whole  disappears.  It  is 
difficult  to  account  for  all  these  changes  in  the  same 
cloud,  unless  we  attribute  it  to  the  different  changes 
in  its  electricity,  by  which  it  is  supposed  to  be  com- 
pounded. 

The  motion  of  clouds  is  most  frequently  directed 
by  the  currents  of  air,  though  this  is  not  always  the 
case,  particularly  when  thunder  is  expected ;  that  is, 
when  the  clouds  are  highly  charged  with  electricity, 
then  they  are  observed  to  move  very  slowly,  and  some- 
times appear  quite  stationary.  In  some  cases  the  motion 
of  the  clouds  evidently  depends  on  their  electricity, 
independently  of  any  current  whatever ;  for,  in  calm 
and  warm  weather,  small  clouds  are  often  seen  moving 
in  different  directions,  setting  out  and  meeting  each 
other  at  such  short  distances  as  to  make  it  impossible 
that  they  should  be  governed  by  different  currents  of 
air.  When  clouds  of  this  description  join  each  other, 
they  do  not  form  a  larger  cloud ;  but,  on  the  contrary, 
become  less,  and  sometimes  totally  vanish,  which  is 
supposed  to  arise  from  the  discharge  of  the  different 
electricities  with  which  they  were  charged. 

This  appears  to  throw  a  new  light  on  the  formation 
of  clouds;  for  if  two  clouds,  one  electrified  positively, 
and  the  other  negatively,  destroy  each  other  on  con- 
tact, it  may  follow  that  any  quantity  of  vapour  in  the 
atmosphere  is  invisible ;  unless  it  be  electrified  either 
positively  or  negatively,  when  it  will  be  seen  as  a 
cloud. 

T 


158  . 

Hall 

As  snow  is  formed  by  a  congelation  of  vapour  in 
the  upper  regions  of  the  atmosphere  by  an  extraor- 
dinary degree  of  cold,  we  may  fairly  suppose,  from 
the  appearance  of  hail,  that  it  is  a  congelation  of  a  like 
kind  arising  from  cold,  but  in  a  lower  medium,  as  the 
descending  drops  of  rain  pass  through  it,  which  forms 
them  into  a  kind  of  rarefied  ice.  But,  however,  this 
subject,  like  a  great  many  others  in  meteorology,  can- 
not be  explained  in  a  very  satisfactory  manner. 

Beccaria  imagines  that  hail  is  formed  in  the  higher 
regions  of  the  air,  where  the  cold  is  intense,  and  where 
the  electric  matter  is  very  copious,  by  which  a  num- 
ber of  aqueous  particles  are  frozen,  and  that  these  par- 
ticles collect  others  in  their  descent,  so  that  the  den- 
sity of  the  substance  of  the  hailstones  grows  less  and 
less  from  the  centre,  that  being  the  hardest  which  is 
first  formed  in  the  upper  regions  of  the  atmosphere. 
He  likewise  adds,  in  support  of  this  principle,  that  the 
size  of  hailstones,  like  drops  of  rain,  is  the  smallest  on 
the  tops  of  high  mountains,  which  corresponds  with 
the  observation  of  different  persons  that  have  observed 
the  descent  of  hail  in  such  situations. 

Lightning  and  Thunder, 

It  has  been  sufficiently  ascertained  by  experiment, 
that  lightning  is  the  same  as  electricity,  and  that  thun- 
der clouds  are  charged  in  a  positive  and  negative  state, 
and  that  they  will  even  change  this  state  many  times 
in  an  hour;  but  whether  this  fluid  be  generated  or  col- 
lected in  the  atmosphere,  is  q, question  that  has  not  yet 
been  properly  resolved. 

Beccaria  is  of  opinion,  that  the  clouds  receive  their 
electricity  from  the  earth;  for,  as  clouds  are  formed 
from  exhaled  vapours,  the  same  power  in  nature  which 
attracts  them  from  the  earth  draws  them  towards  each 
other,  so  that  a  kind  of  aqueous  conducting  chain  is 


Lightning  and  Thunder,  159 

formed  for  the  passage  of  the  fluid  from  its  grand  re- 
ceptacle the  earth.  Thus  may  electricity  be  transmit- 
ted from  one  part  of  the  earth  that  is  surcharged,  to 
another  that  is  deficient.  The  quantity  contained  by 
the  clouds  is  in  such  abundance,  that  although  parti- 
cular clouds  are  perpetually  discharging  the  collected 
matter  to  the  earth,  yet  the  unbounded  stores  of  the 
earth  instantly  supply  the  deficiency,  so  that  the  clouds 
continue  to  discharge  their  electricity,  with  a  short 
intermission,  during  the  whole  time  of  a  thunder- 
storm, or  till  the  electricity  be  restored  to  an  equili- 
brium. 

The  rumbling  noise  of  thunder  which  follows  the 
flash  or  discharge,  most  probably  arises  from  the  col- 
lapsing of  the  air  w  hich  is  rarefied  in  the  electric  cir- 
cuit, so  that  the  sound  is  soonest  heard  from  that  part 
of  the  direction  which  is  nearest  to  us,  and  continues 
successively  according  to  .the  distances. 

Some  imagine  that  the  clouds  do  not  receive  their 
electricity  from  the  earth,  but  from  the  heating  and 
cooling  of  the  air;  so  that  the  clouds  in  passing 
through  a  rarefied  part  of  the  atmosphere,  receive 
electricity  from  it,  and  give  it  back  again  to  those  parts, 
which  are  in  a  more  condensed  state.  Others  have 
conceived,  from  the  sultry  state  of  the  atmosphere  in 
thunder  storms,  that  electric  matter  is  generated  by 
the  fermentation  of  sulphurous  vapours  with  mineral 
or  acid  vapours  in  the  atmosphere. 

Whatever  may  be  the  cause  that  disturbs  the  na- 
tural equilibrium  of  the  air,  or  the  means  by  which  it 
restores  itself,  the  concussion  of  the  elements  at  least 
proves  the  inequality  of  the  atmosphere;  and  the 
dreadful  consequences  which  frequently  follow  it, 
make  every  one  anxious  to  guai'd  against  its  effects. 

The  Aurora  Borealis,  or  Northern  Lights,  are  sup- 
posed to  be  produced  by  the  continual  discharge  of 
electric  fluid;  for,  in  the  higher  and  more  attenuated 


16G  Lightning  and  Thunder. 

parts  of  the  atmosphere  where  they  are  always  seen,  a 
large  quantity  of  the  fluid  cannot  be  collected  by 
clouds  to  make  a  great  concussion  like  thunder;  but 
it  is  dispersed  as  fast  as  it  is  collected,  which  gives  that 
perpetual  flashing  resembling  the  appearance  of  elec- 
trical fluid  in  a  rarefied  receiver. 

What  are  called  Falling,  or  Shooting  Stars,  are  sup- 
posed to  be  produced  by  electrical  fluid  passing 
through  the  air  when  its  course  is  not  disturbed  by 
stormy  clouds ;  and  by  attaching  itself  to  those  con- 
ductors it  may  meet  with  in  the  air,  it  becomes  visible 
in  its  passage  to  the  earth. 

The  Ignis  Fatuus  is  a  luminous  body  which  is  seen 
in  the  dark  hovering  over  bogs  and  marshy  places, 
seemingly  both  changeable  in  situation  and  varying 
in  its  appearances.  This  body  is  generally  considered 
as  inflammable  air  arising  from  the  marsh,  ignited  by 
electricity. 

Even  earthquakes  are  now  supposed  to  be  produced 
by  electricity;  what  was  once  thought  to  proceed 
from  subterranean  vapours,  is  now  attributed  to  the 
discharge  of  a  cloud  in  a  highly  electric  state,  either 
in  its  passage  to  another  cloud  or  to  the  earth.  Earth- 
4  quakes  are  most  frequent  in  dry  and  hot  countries, 
and  hot  climates  are  more  subject  to  lightning  and 
other  electrical  meteors,  than  those  which  are  more 
remote  from  the  equator.  Another  circumstance  which 
seems  to  confirm  this  opinion  is,  that  the  concussion 
of  an  earthquake  is  not  followed  by  any  noxious  smell, 
which  would  probably  take  place  if  it  proceeded  from 
the  explosion  of  vapour;  and  the  atmosphere  is  found 
to  be  highly  charged  with  electricity  for  some  time 
previous  to  this  dreadful  calamity. 

Much  of  the  power  of  electricity  still  depends  upon 
hypothesis,  and,  although  great  advances  have  been 
recently  made,  yet  much  remains  to  be  done;  we  still 
see  through  a  veil,  but  if  ever  time  should  withdraw 


this  curtain  from  before  our  eyes,  then  the  wonders  of 
electricity  will  be  completed ;  we  may  then  discover 
its  effects,  not  only  in  the  laws  of  planetary  motion, 
but  Hkewise  trace  it  to  its  source  in  meteorology, 
magnetism,  vegetation,  muscular  motion,  and  all  the 
economy  of  nature. 

mnd. 

Wind  is  a  current  of  air  which  usually  blows  from 
one  part  of  the  horizon  to  its  opposite  part. 

Winds  may  be  reduced  into  three  classes,  called  va- 
riable, periodical,  and  general. 

Variable  winds  are  those  w^hich  are  not  subject  to 
any  particular  period,  either  in  duration  or  return. 

The  stated  or  periodical  winds  are  such  as  return 
at  certain  times ;  these  may  be  divided  into  two  kinds, 
viz.  the  sea  and  land  breezes,  which  are  produced  by 
the  diurnal  motion  of  the  sun,  and  blow  alternately 
from  the  sea  and  land ;  and  monsoons,  which  are 
caused  by  the  annual  revolution  of  the  sun,  blowing 
one  way  for  a  certain  number  of  months,  and  the  op- 
posite way  for  the  rest  of  the  year. 

General  winds,  which  are  usually  called  trade 
winds,  blow  always  nearly  in  the  same  direction,  as 
those  between  the  tropics  in  the  Atlantic  and  Pacific 
Oceans. 

The  annual  and  diurnal  revolution  of  the  sun  may 
be  the  general  cause  of  winds,  but  this  hypothesis  can 
by  no  means  sufficiently  explain  the  phenomena, 
without  having  recourse  to  some  other  aid,  as  these 
causes  could  only  produce  regular  winds,  the  pro- 
gress of  which  would  correspond  and  be  connected 
with  the  seasons. 

With  respect  to  the  effect  of  heat  on  the  air,  there 
is  no  doubt  but  that  those  places  which  receive  the 
^eatest  force  from  the  sun's  rays,  will  have  the  air 


162  IVind, 

more  rarefied,  and  its  elasticity  more  weakened,  than 
those  which  receive  less  of  his  influence;  therefore  a 
wind  will  blow  towards  the  rarefied  place,  as  the  re- 
sistance will  not  be  able  to  oppose  the  adjoining  pres- 
sure. For  the  spring  of  the  air  increases  as  the  com- 
pressing weight  increases,  and  compressed  air  is 
denser  than  that  which  is  less  compressed,  so  ihat  all 
winds  will  blow  into  a  rarer  air  out  of  that  place  which 
is  filled  with  a  denser.  This  principle  is  practically  de- 
monstrated by  the  current  of  air  that  rushes  in  on  all 
sides  to  a  fire  burning  in  an  open  situation;  here  the 
particles  of  air  that  surround  it,  being  rarefied  by  the 
heat,  ascend  in  a  constant  current,  and  the  cooler  or 
denser  air  presses  in  by  its  elasticity,  and  produces 
a  very  sensible  effect  on  the  bodies  of  those  that  are 
placed  at  a  small  distance  from  the  fire. 

Dr.  Hal  ley,  who  has  paid  great  attention  to  this 
subject,  supposes  that  it  is  the  action  of  the  sun's 
beams  that  produces  the  general  winds,  as  it  passes 
over  the  air^  earth,  and  water;  for,  as  the  sun  appears 
to  be  continually  shifting  to  the  Westward  during  a 
diurnal  fevoiution,  the  lower  air  becomes  attenuated 
by  his  rays,  and  the  tendency  of  the  whole  body  is  to- 
wards this  rarefied  passage,  which  produces  an  easterly 
wind  to  about  30  degrees  on  each  side  of  the  equator; 
and  as  this  motion  is  communicated  to  a  vast  ocean 
of  air,  the  current  continues  during  the  night,  till  the 
sun  appears  again  to  give  fresh  impulse,  and  restore 
the  motion  that  was  lost  in  its  absence,  which  causes 
the  easterly  wind  to  be  perpetually  in  this  situation. 
As  the  air  towards  the  poles  is  less  rarefied  than  that 
between  the  tropics,  it  will  necessarily  follow  that  a 
wind  will  blow  from  the  north  and  south  towards  the 
equator. 

If  the  surface  of  the  globe  were  covered  with  water 
alone,  the  winds  would  be  perpetually  the  same  as 
they  are  in  the  Atlantic  and  Pacific  Oceans ;  but  tlit 


Wind,  163 

large  continents  of  land  receiving  a  greater  degree  of 
heat  than  the  water  during  the  day,  they  communicate 
it  to  the  air  above  them,  which  becoming  more  rare- 
fied than  that  part  over  the  sea,  the  denser  air  passes 
towards  the  land.  This  accounts  foif  those  westerly 
instead  of  easterly  winds  that  blow  towards  the  coast 
of  Guinea  from  some  distance  on  the  sea. 

The  sea  and  land  breezes  in  the  West  Indies  no 
doubt  arise  from  the  same  cause.  The  breeze  from 
the  sea  to  the  land  begins  to  appear  about  nine  o'clock 
in  the  morning,  and  keeps  gradually  increasing  till 
noon,  and  dies  away  about  four  or  five  in  the  after- 
noon ;  about  six  in  the  evening  it  changes  into  a  fand 
breeze,  which  continues  blowing  towards  the  sea  till 
near  eight  the  next  morning. 

These  changes  may  be  accounted  for  according  to 
the  preceding  principles;  for,  as  the  heat  takes  more 
effect  on  the  land  than  on  the  water  during  the  day, 
the  air  over  the  land  becomes  more  rarefied,  which 
causes  the  cooler  air  to  rush  in  to  keep  up  the  equili- 
brium. In  the  evening,  as  soon  as  the  sun  is  set,  the 
dews  come  on  so  excessively,  that  the  air  becomes 
suddenly  cooled,  consequently  more  dense  than  that 
which  is  over  the  water,  and  this  causes  the  air  to 
press  from  the  land  to  the  water,  which  produces  the 
opposite  current. 

The  cause  of  the  monsoons,  or  periodical  winds,  is 
owing  to  the  course  of  the  sun  northwards  of  the  equa- 
tor one  half  of  the  year,  and  southwards  the  other. 
While  it  passes  through  the  six  northern  signs  of  the 
ecliptic,  the  various  countries  of  Arabia,  Persia,  India, 
and  China,  are  heated,  and  reflect  great  quantities  of 
the  solar  rays  into  the  atmosphere,  by  which  means  it 
becomes  greatly  rarefied,  and  the  equilibrium  is  de- 
stroyed; in  restoring  it  again,  the  air  not  only  rushes 
in  from  the  equatorial  parts  southwards  where  it  is 
colder,  but  likewise  that  from  the  northern  climes 
must  necessarily  have  a  tendency  towards  those  parts, 
which  produces  the  monsoons  for  the  first  six  months. 


164  PFind. 

Then,  during  the  other  six  months,  whilst  the  sun 
is  traversing  the  ocean,  and  countries  in  the  southern 
tropic,  it  heats  and  rarefies  the  air  in  those  parts,  con- 
sequently causes  the  equatorial  air  to  alter  its  course, 
to  veer  quite  about,  and  blow  from  the  opposite  points 
of  the  horizon. 

These  are  the  general  affections  of  constant  and 
regular  winds ;  but  the  whole  .ire  subject  to  variations 
and  exceptions,  on  account  of  different  circumstances, 
local  or  otherwise. 

Some  philosophers  differ  from  Dr.  Halley  in  his 
theory  of  the  wind,  and  suppose  that,  as  the  earth  turns 
on  its  axis  eastwards,  the  particles  of  air  being  very 
light  are  left  behind,  so  that  with  respect  to  the  earth's 
surface  they  would  seem  to  move  westerly,  which 
produces  a  constant  easterly  wind,  which  is  the  strong- 
est and  most  regular  where  the  diurnal  motion  is  the 
swiftest,  that  is  between  the  tropics. 

Others  conceive  that  the  atmosphere  is  a  gravitating 
fluid  substance,  subject  to  the  attraction  of  the  sun  and 
moon  as  well  as  the  earth;  and,  therefore,  when  their 
influence,  either  singly  or  conjointly,  is  opposed  to 
that  of  the  earth,  the  same  effects  will  take  place  in  the 
air  as  are  produced  upon  water;  and  that  there  are 
tides  of  air  which  vary  in  their  form  and  pressure,  ac- 
cording to  the  different  positions  of  the  attracting  bo- 
dies. But  that,  as  various  altitudes  of  the  atmosphere 
may  have  an  equality  of  weight  or  pressure,  we  can- 
not discover  the  aerial  tides  of  ebb  and  flow  by  the 
barometer;  yet  the  change  which  is  produced  by  the 
difference  of  pressure,  must  create  a  motion  in  the 
parts  which  produces  wind,  either  more  or  less,  as 
these  differences  conspire  with,  or  act  against,  each 
other. 

The  force  or  velocity  of  the  most  vehement  wind 
does  not  fly  at  the  rate  of  more  than  fifty  or  sixty  miles 
kn  hour,  and  the  medium  velocity  of  wind  is  not  more 
than  twelve  or  fifteen  milejs  an  hour.  Sometimes  thr 


Wind.  165 

wind  is  so  slow  as  not  to  exceed  the  velocity  of  a  per- 
son walking  or  riding  in  it;  in  this  case  a  person  mov- 
ing with  it  feels  no  wind,  as  there  is  no  difference  of 
velocity,  or  no  relative  force,  which  is  all  we  are  sen- 
sible of  whilst  the  body  is  in  motion. 


U 


106 


LIGHT  AND  COLOURS. 

Light  is  that  power  by  which  objects  are  made 
perceptible  to  our  sense  of  seeing,  or  the  sensation  oc- 
casioned in  the  mind  by  the  view  of  luminous  objects. 

Light,  like  many  other  effects  in  nature,  the  princi- 
ple and  essence  of  which  exceed  the  bounds  of  human 
understanding,  has,  for  many  ages,  been  a  subject  of 
much  speculation  and  hypothesis;  it  has  been  consi- 
dered as  a  mere  quality  of  particular  bodies,  or  a  fluid 
medium  by  which  the  vibrations  of  luminous  bodies 
are  carried  to  the  eye ;  but  Newton  demonstrates  it  to 
be  an  absolute  body,  composed  of  infinitely  small 
particles  of  matter,  which  issue  by  a  repulsive  force 
from  luminous  bodies  with  wonderful  velocity,  di- 
verging in  right  lines  in  all  directions. 

The  particles  of  light  issue  from  the  sun  as  the  pri- 
mary source,  and  keep  a  rectilinear  motion,  till  they 
are  inflected  by  the  attraction  of  some  other  body,  or 
refracted  by  passing  obliquely  through  a  medium  of 
different  density,  or  reflected  by  the  intervention  of 
an  opposing  body ;  so  that  the  small  particles,  of  which 
light  is  composed,  are  governed  by  the  power  of  at- 
traction in  the  same  manner  as  the  particles  of  the 
grossest  bodies. 

That  the  particles  of  light  are  infinitely  small,  may 
be  reasonably  inferred  from  their  penetrating  the 
densest  bodies;  the  pores  of  glass,  crystal,  or  a  dia- 
mond, cannot  stop  the  subtilty  of  light;  and  yet  the 
greatest  collection  of  this  matter  has  never  been  found 
to  have  any  sensible  weight,  even  in  the  finest  scale. 
Considering  the  immense  velocity  of  light,  if  the  par- 
ticles were  not  infinitely  small,  and  probably  placed 
at  a  considerable  distance  from  one  another,  the  pres- 
sure on  the  tyt  would  be  insufferable. 


Light.  167 

It  appears  evident,  that  the  rays  of  light  are  emitted 
in  right  lines,  from  the  shadow  which  is  thrown  be- 
hind  those  bodies  on  which  they  fall,  as  the  corre- 
sponding parts  of  the  substance  and  shadow  form  right 
lines  with  the  source  of  the  ray. 

The  velocity  of  light  has  been  considered  instan- 
taneous; nor  can  its  passage  from  one  visible  object 
to  another  be  marked  by  any  difference  of  time,  al- 
though the  following  discovery  of  Roemer's,  sup- 
ported by  Cassini  and  others,  sufficiently  proves  a  pro- 
gressive motion.  He  observed  that  the  eclipses  of 
Jupiter's  satellites  varied  16 2  minutes  in  time  in  some 
particular  situations  of  the  earth  in  its  orbit,  being  81 
minutes  sooner  than  the  calculated  time  Avhen  the 
earth  wSHP^^Rarest  to  the  planet,  and  8t  minutes  later 
than  the  tables  when  the  earth  was  in  the  opposite 
part  of  its  orbit.  From  this  observation,  Cassini,  and 
others,  have  concluded  that  the  difference  of  time 
proceeds  from  the  progressive  motion  of  light  in 
passing  the  orbit  of  the  earth.  This  subject  may  be 
further  explained  by  the  following  diagram. 


168 


Light, 


B 


Let  A  be  the  sun,  or  the  centre  of  the  system,  from 
which  the   tables   are  calculated; 
B  c  the  earth's  orbit;    e  Jupiter, 
and  D  one  of  his  satellites  just  en- 
tering his  disk.    Then  an  observer 
at  A  would  find  the  time  of  im- 
mersion coincide  with  the  tables ; 
but  at  B  it  takes  place  8i  minutes 
sooner  than  at  a,  and  at  c  8i  mi- 
nutes later;  which,  taken  together, 
gives  a  difference  of  16 J  minutes 
for  the  progressive  time  of  the  re- 
flected light  of  the  satellite  in  pass- 
ing from  B  to  c,  the  diameter  of    / 
the  earth's  orbit.    Now  if  we  take  / 
the  diameter  of  the  earth's  orbit  at  I 
190  millions  of  miles,  and  divide  \ 
it  by  990,  the  seconds  in  16i  mi-    \ 
nutes,    the    result   will   be   about 
.200000  miles,  which  is  the  won- 
derful velocity  of  light  in  a  second  of  time. 

As  the  sun  is  placed  in  the  centre  of  the  earth's 
orbit,  the  distance  of  these  two  bodies  from  each 
other  is  equal  to  the  semidiameter  of  the  earth's  or- 
bit, or  95  millions  of  miles ;  so  that  the  particles  of 
light  aie  transmitted  from  the  sun  to  the  earth  in 
8t  minutes,  which  is  near  two  millions  of  times  swifter 
than  the  velocity  of  a  cannon  ball,  supposing  it  to  fly 
at  the  rate  of  450  miles  in  an  hour. 

Natural  philosophers  were  formerly  of  opinion  that 
the  solar  light  was  simple  and  uniform,  without  any 
difference  or  variety  of  part^  and  that  the  different 
colours  of  objects  were  made  by  refraction,  reflection, 
or  shadows.  But  the  illuminated  mind  of  Newton  has 
demonstrated,  from  clear  and  decisive  experiments, 
that  light  is  not  similar,  but  compounded  of  dissimi- 
lar rays,   some  more  and  some  less  refrangible,  or 


#A 


•^c" 


Prism.  169 

capable  of  being  turned  out  of  their  course ;  and  that 
those  rays  which  are  the  most  refraiigible  are  like- 
wise the  most  reflexible,  or  most  easily  turned  back, 
and  accordingly  as  the  rays  differ  in  these  qualities 
they  excite  in  us  the  sensation  of  different  colours. 

To  explain  this  subject  more  fully:  the  sun's  light 
consists  of  rays  which  have  a  considerable  inequality 
of  refraction  as  they  are  transmitted  through  the  me- 
dium of  the  atmosphere,  and  impress  a  sensation  of 
colours  as  they  are  more  or  less  bent  in  their  course, 
which  we  call  violet,  indigo,  blue,  green,  yellow, 
orange,  and  red.  Those  rays  which  pass  the  most 
directly,  or  are  the  least  refrangible,  produce  red; 
the  next  rays  in  refrangibility  produce  orange,  then 
yellow,  &c.  The  rays  which  produce  violet  are  the 
most  broken,  or  have  the  greatest  refrangibility. 

These  are  all  the  primary  colours  in  nature;  but  by 
blending  or  mixing  different  rays  together  they  mu- 
tually alloy  each  other,  and  constitute  intermediate 
colours,  and  shades  of  colours,  of  every  description. 

Whiteness  is  a  peculiar  production;  it  is  not  com- 
posed of  any  particular  ray  of  light,  but  produced  by 
a  copious  reflection  and  due  proportion  of  the  rays  of 
all  sorts  of  colours. 

Blackness  proceeds  from  the  peculiar  quality  of  a 
body  whi^h  stifles  and  absorbs  the  rays  of  light  that 
fall  upon  it;  so  that  instead  of  reflecting  them  out- 
wards, they  are  reflected  and  refracted  inwards,  till 
the  incident  rays  are  lost. 

Prism, 

The  principle  of  colours  is  beautifully  illustrated 
and  explained  by  what  is  called  a  prism,  which  is  a 
solid  piece  of  glass,  with  three  flat  sides,  through 
which  the  sun's  rays  are  refracted. 


170 


Prism. 


Let  ABC  represent  a  glass  prism,  and  e  a  small 
hole  in  the  window-shutter  of  a  darkened  room,  by 
which  the  pencil  of  rays  e  g  enters  and  falls  on  the 
side  of  the  prism  at  c.  If  the  medium  of  the  glass  did 
not  obstruct  the  rays,  thdf'^  would  pass  on  in  a  straight 
line  and  parallel  direction  to  h,  and  there  illuminate 
a  small  circle  in  the  side  of  the  room  h  i  ;  but  when 
the  rays  enter  and  pass  out  of  the  denser  medium  of 
the  glass  they  are  refracted  or  bent  towards  l  ;  there- 
fore if  the  rays  were  equally  refracted  by  the  prism, 
they  would  pass  on  from  f  to  l  in  parallel  lines,  and 
enlighten  a  circular  spot  at  l,  similar  to  that  which 
was  formed  at  h.  But  if  the  pencil  of  liglijb.  be  com- 
posed of  rays  which  are  not  equally  refrangible,  then 
those  which  are  the  least  refrangible  windfall  near- 
est to  the  right-lined  direction  e  g  h,  and* hose  that 
are  the  most  refrangible  will  be  the  most  distant,  and 
tl^e  intermediate  degrees  of  refrangibility  will  issue  in 
different  rays  between  the  two  extremes.  This  ac- 
cords with  experiment;  for  after  the  rays  quit  the 
side  of  the  prism  f,  they  diverge  according  to  their 
refrangibility,  and  form  an  oblong  spectrum,  va- 
riously coloured,  on  the  side  of  the  wall  between  i 
and  k;  the  lower  part  of  which,  being  the  least  bent, 
prodtit^es  a  lively  red  colour;  this  changes  by  grada- 
tion into  an  orange,  thence  into  a  yellow,  and,  as  the 
rays  rise  higher,  into  a  green,  blue,  indigo,  and  vio- 


Prism.  171 

let,  which  is  the  most  distant,  as  being  the  most 
broken. 

There  is  a  remarkable  analogy  between  colour  and 
sound;  for  it  is  found,  that  the  divisions  of  the  un- 
compounded  colours  on  the  spectrum  agree  with  the 
different  divisions  of  a  musical  chord. 

As  an  additional  proof  that  the  refrangibility  of  the 
sun's  rays  is  various,  and  that  the  different  rays  re- 
flect their  own  colours ;  if  the  spectrum  be  received 
on  a  perforated  board,  so  that  the  uncompounded  co- 
lours may  pass  distinctly  through  the  hole,  it  will  be 
found  that  they  still  preserve  their  individual  colour, 
whether  it  be  received  on  a  sheet  of  white  paper  be- 
hind the  hole,  or  be  again  refracted  by  another  prism 
upon  some  other  surface. 

The  prism  likewise  shows,  that  those  rays  which 
are  the  most  refrangible  are  also  the  most  reflexible. 

For  if  the  prism  a  b  e  be  placed  in  a  darkened 
room,  in  such  a 
manner  that  the 
pencil  of  light 
which  passes 
through  the  hole 
D  may  be  re- 
flected from  the 
point  E  in  the 
base;  the  violet 
rays  will  be  first 
seen  reflected  in  the  upper  line  e  f,  and  the  other 
rays  will  continue  their  refraction  through  e  c  and 
E  G,  Sec;  but  if  the  prism  be  gently  moved  on  its 
axis,  the  indigo  will  be  reflected  after  the  violet,  then 
the  blue,  green,  and  so  on,  till  the  red  be  reflected, 
which  is  the  last. 

Now,  as  the  rays  of  light  differ  both  in  refrangibi- 
lity and  reflexibility,  in  accounting  for  the  different 
colours  of  bodies,  it  is  supposed  that  different  bodies 
are  endued  with  a  power  or  aptitude  to  reflect  the 


172'  Prism. 

rays  of  one  particular  colour,  and  to  imbibe  the  rest. 
This  opinion  is  likewise  supported  by  experiment; 
for,  if  any  body  be  placed  in  the  uncompounded  light 
of  the  spectrum,  so  that  a  pure  and  distinct  colour 
may  fall  upon  it,  the  body  will  appear  of  the  same 
colour;  only  with  this  difference,  that  it  will  reflect 
that  colour  most  brightly,  which  is  the  same  as  the 
body  itself  reflects  in  full  light.  But  as  all  bodies  re- 
flect other  colours,  in  some  degree,  besides  the  pre- 
dominant colour  which  belongs  to  them,  they  cannot 
appear  so  full  and  clear  as  the  colour  is  seen  in  the 
spectrum  where  it  is  uncompounded;  but  they  reflect 
their  colour  feebly  and  weak  in  proportion  to  the 
Compound  of  the  rays  which  are  reflected. 


173 


OPTICS. 


This  science,  in  its  extended  signification,  em- 
braces a  considerable  part  of  the  philosophy  of  na- 
ture ;  connected  with  vision,  it  not  only  comprehends 
the  whole  doctrine  of  light  and  colours,  but  extends 
to  the  phenomena  of  visible  objects  in  general. 

The  word  optics  is  here  taken  in  a  stricter  sense, 
and  implies  an  explanation  of  that  part  of  vision 
which  causes  objects  to  produce  different  effects  on 
the  eye ;  why  they  appear  further  off,  or  nearer,  than 
they  really  are ;  and  why  they  appear  more  or  less 
distinctly  by  the  modifications  and  inflections  of  the 
rays  of  light  in  passing  through  different  mediums. 

This  subject  is  divided  into  two  parts,  called  Re- 
flected and  Refracted  Vision. 


X 


174 


Refracted  Vision, 

This  shows  the  different  directions  and  effects  of 
rays  of  light  in  passing  from  one  medium  into  ano- 
ther; which  appai'ently  increases  the  magnitude  of  a 
body,  brings  distant  objects  nearer  to  the  sight,  and 
makes  those  minute  parts  of  nature  visible  that  the 
unassisted  eye  could  never  discover. 

Rays  or  pencils  of  light  pass  in  right  lines  from  lu- 
minous objects,  each  carrying  an  impression  to  the 
eye  of  that  part  of  the  object  whence  it  was  emitted. 
When  the  direction  of  the  ray  keeps  in  the  same  me- 
dium, the  object,  image,  and  eye,  will  be  in  a  straight 
line ;  but  when  the  ray  is  transmitted  obliquely  through 
different  mediums,  such  as  air,  water,  glass,  or  any 
other  transparent  body,  as  it  enters  into  each  it  changes 
its  direction,  inclining  more  or  less  to  the  perpendi- 
cular of  the  medium,  according  to  the  density  of  the 
body.  This  deflection  is  supposed  to  proceed  from  the 
attraction  of  the  denser  medium,  which  acts  in  right 
lines  perpendicular  to  the  surface;  thus  the  attractive 
j)Ower  impedes  the  rays  in  their  oblique  course,  and 
draws  them  towards  the  axis  of  the  medium;  when 
rays  fall  perpendicular  they  have  no  refraction. 

Definitions, — The  diagram  d  h  e  represents  a  ves- 
sel filled  with  water,  or  any 
transparent  body  denser  than 
air,  and  d  b  e  is  its  surface. 
The  line  a  b  is  the  course  of 
the  ray  of  light  from  the  object 
or  radiant  at  a,  which  is  called 
the  line  of  incidence,  and  b  is 
the  point  of  incidence ;  the  line 
G  B  is  the  line  of  refraction,  or 
the  course  of  the  ray,  when  the  ^ 

direction  is  changed  at  b,  by  entering  into  a  denser 


A 

C 
B 

E 

V 

r 

\j/^ 

Refracted  Visiofi.  175 

medium  than  that  of  the  air  through  wliich  it  has 
passed;  the  right  line  d  e,  or  the  surface  of  the  me- 
dium, where  the  lines  of  incidence  and  refraction 
meet  each  other,  is  called  the  plane  of  refraction ;  b  h  , 
the  perpendicular  to  that  plane,  is  the  axis  of  refrac- 
tion ;  and  the  continuation  of  the  same  line,  c  b, 
above  the  surface  of  the  medium,  is  called  the  axis  of 
incidence.  The  angle  a  b  c,  which  is  formed  by  the 
line  of  incidence  and  the  perpendicular  c  b  ,  is  called 
the  angle  of  incidence  ;  and  a  c  is  the  sine  of  the  an- 
gle of  incidence.  The  angle  h  b  g  ,  which  is  formed 
by  the  line  of  refraction  and  its  axis,  is  the  angle  of 
refraction ;  k  g  is  its  sine  ;  and  the  angle  g  b  f,  which 
is  the  difference  between  the  angles  of  incidence  and 
refraction,  is  called  the  angle  of  deviation. 

The  course  of  rays,  or  pencils  of  light,  is  divided 
into  three  kinds,  viz.  parallel,  converging,  and  di- 
verging rays. 

Bays  are  called  parallel,  when  they  would  pass  to 
infinity  at  equal  distances  from  each  other,  as  n  l  o  m. 
When  rays  of  light  issue  from  bodies  at  immense 
distances,  they  are  considered 
as  parallel  to  one  another ;  so 
that  those  rays  which  proceed 
from  the  sun  and  pass  through 
our  atmosphere,  are  taken  as 
parallel,  though  each  point  of  K 
the  sun  is  a  radiant,  diverging 
rays  of  light  on  the  earth;  but 
as  the  distance  is  so  immense, 
and  the  angle  of  divergency  so  infinitely  small,  the 
rays  may  be  fairly  considered  as  passing  in  the  same 
parallelism. 

Converging  rays  are  those  which,  in  passing  from 
a  rare  to  a  denser  medium,  are  refracted  or  bent 
towards  the  perpendicular,  and  meet  in  a  common 
centre  called  the  focus,  as  n  p  o. 

Diverging  rays  recede  from  the  axis  or  perpendi- 


176 


Refracted  Vision. 


cular,  in  passing  from  a  dense  into  a  rarer  medium ; 
or  when  converging  rays  have  crossed  each  other  in 
their  focus,  or  burning  point,  then  they  diverge  or 
pass  off  from  one  another,  as  q^  p  r  represents  in  the 
figure. 

A  ray  of  light  in  its  oblique  passage  out  of  a  rare 
into  a  denser  medium,  as  from  air  into  water,  is  re- 
fracted towards  the  perpendicular  of  the  water;  and 
the  angle  of  incidence  is  alvcjys  greater,  in  a  given 
ratio,  than  the  angle  of  refraction,  except  when  the 
ray  falls  perpendicular  to  the  denser  medium,  then 
they  become  equal. 

If  a  ray  of  light,  which  passes  from  a  in  air,  fall  on 
the  denser  medium  of  the 
water  b,  it  does  not  pro- 
ceed in  a  right  line  to  i ;  but 
is  attracted,  or  drawn  out 
of  its  course,  towards  the 
perpendicular  b  d,  and  pas- 
ses in  the  direction  b  g, 
which  is  nearer  to  the  axis 
than  the  straight  line  i  b  . 

If  G  B  be  a  ray  of  light 
passing  from  a  denser  into  a 

rarer  medium,  or  from  water  into  air,  it  will  not  pro- 
ceed in  a  right-lined  direction  to  k  ;  but  when  it 
comes  to  b  at  the  surface,  its  course  wi]'  be  refracted, 
and  the  ray  will  recede  from  the  perpendicular  d  c, 
and  pass  on  in  the  direction  b  a  . 

It  may  be  practically  found,  by  the  following  ex- 
periment, that  light,  in  passing  out  of  a  rare  into  a 
denser  medium,  approaches  nearer  to  the  perpendi- 
cular, and  that  it  recedes  in  passing  from  a  denser  to 
a  rarer. 


Refracted  Fision, 


177 


A  piece  of  money  being  placed  at  the  bottom  of  the 
cylindrical  vessel  o  r,  let  the  ]^  (y 

eye  of  the  observer  be  so 
placed  at  N,  as  just  to  lose 
sight  of  the  object  at  p ;  or 
so  that  a  ray  shall  pass  from 
the  remote  part  of  the  piece 
to  the  eye  in  the  direction 
PON.  Whilst  the  eye  con- 
tinues in  this  situation,  if  the 
vessel  be  filled  with  water  the  object  will  become  vi- 
sible; for  the  rays  which  pass  from  s  in  a  right  line 
will  be  bent  at  o,  the  surface  of  the  water,  and  fall 
into  the  eye  at  n,  or  be  refracted  from  the  perpendi- 
cular o  q.  It  may  here  be  necessary  to  observe,  that 
impressions  are  received  in  the  eye  by  the  rays 
which  proceed  from  the  object;  so  that  in  the  above 
experiment,  when  the  vessel  is  filled  with  water,  the 
rays  pass  from  a  denser  into  a  rarer  medium,  conse- 
quently recede  from  the  axis  of  refraction  when  they 
enter  the  eye.  If  we  consider  the  ray  as  issuing  from 
N,  that  is,  from  the  rare  into  the  denser  medium,  it 
will  pass  in  the  direction  o  s,  approaching  the  per- 
.pendicular ;  and,  as  the  water  is  drawn  out  of  the 
vessel,  the  ray  will  recede  till  it  falls  into  the  straight 
line  NOP,  at  the  outer  edge  of  the  coin. 

The  sine  of  the  angle  of  incidence  is  in  a  given 
ratio  to  the  sine  of  the  angle  of  refraction ;  this  may 
be  found  experimentally  in  the  following  manner. 


178 


Refracted  Vision. 


Let  the  quadrant  c  d  e,  which  is  graduated  on  its 
circular  edge,  have  two  moving 
indices,  a  and  b,  that  turn  in 
the  point  e,  and  let  a  e  be  pro- 
longed to  F ;  then  set  the  index 
A  to  15  degrees  on  the  scale, 
and  B  to  19^°,  and  bring  the 
edge  of  the  quadrant  d  e  to  the 
surface  of  a  vessel  of  clear  wa- 
ter, immersing  the  lengthened 
part  of  the  index  a  f  ;  now  the 
immersed  part  f  e  will  appear 
bent  to  G,  which  is  in  a  right  line  with  the  index  b  e. 
In  like  manner  if  a  be  removed  to  30^,  and  b  to  41|<^, 
the  refraction  of  e  f  will  bring  the  apparent  place  of 
the  limb  in  a  right  line  with  b  e.  Thus  the  angle  of 
refraction  may  be  found  to  any  line  of  incidence,  ei- 
ther out  of  a  rare  into  a  denser  medium,  or  from  a 
denser  into  a  rarer;  by  placing  the  longer  index  to 
the  angle  of  incidence  on  the  quadrant,  then  immers- 
ing it  in  water,  and  afterwards  moving  the  shorter 
index  till  it  apparently  coincide  with  the  refracted 
limb  of  the  other,  and  the  number  of  degrees  on  the 
scale,  opposite  to  the  shorter  index,  will  be  the  angle 
of  refraction,  whether  the  incidental  ray  issue  from  a 
denser  or  rarer  medium.  1 

But  as  the  sines  of  the  angles  of  incidence  and  re- 
fraction, between  the  same  media,  are  in  a  constant 
ratio,  if  the  angles  of  incidence  and  refraction  of  one 
incidental  line  be  given,  any  other  may  be  known  by 
finding  the  sines,  and  saying, 

As  the  sine  of  the  known  ^  of  incidence 

Is  to  the  sine  of  its  refraction. 

So  is  the  sine  of  any  other  ^  of  incidence 

To  the  sine  of  its  refraction. 


179 


Reflected  Vision, 


This  part  of  optics  shows  the  effect  produced  by 
rays  of  light  falling  on  a  polished  surface,  and  thence 
returned  to  the  eye,  or  thrown  oif  in  a  certain  direc- 
tion from  the  plane,  without  penetrating  its  substance. 

Rays  of  light  must  either  fall  perpendicularly  or 
obliquely  upon  a  mirror  or  speculum,  if  it  be  per- 
fectly smooth  and  even. 

When  the  ray  falls  perpendicularly  upon  the  sur- 
face, as  D  F  on  A  B,  the 
ray  is  reflected  back 
again  in  the  same  direc- 
tion; but  when  it  falls 
obliquel}^,  like  c  f,  on 
the  plane,  it  is  reflected 
from  the  surface,  and  A 
passes   in    the  direction 

F  E,  making  the  angle  of  reflection  e  f  d  exactly  equal 
to  the  angle  of  incidence  c  f  d. 

This  being  universally  the  law  in  all  oblique  angles 
of  reflected  rays  on  a  polished  surface,  it  may  not  be 
improper  to  note  again,  that,  in  reflected  lights  the 
angle  of  incidence  is  ever  equal  to  the  angle  of  reflec- 
tion^ whether  it  be  in  plane  or  spherical  surfaces^  con- 
cave or  convex. 

Parallel  rays  falling  on  a  plane  mirror  still  keep 
parallel  after  reflection;  for,  as 
the  angle  of  incidence  is  equal 
to  the  angle  of  reflection,  and  as 
the  incidental  rays  e  f  and  h  i, 
are  parallel  to  each  other,  the 
reflected  rays  c  i  and  c  f,  will 
likewise  be  parallel. 

Diverging  rays  have  the  same  divergency  when 


180 


Reflected  Fisioji, 


they  are  reflected,  as  they  would  have  at  an  equal 
distance  if  continued  in  right  lines. 

That  is,  if  two  diverging  rays,  a  g  and  b  h,  be 
reflected  to  e  and  f  ;  the  arc  e  f 
will  be  equal  to  the  arc  d  c ,  which 
would  have  been  the  position 
of  the  rays,  supposing  them  to 
have  passed  in  a  right-lined 
direction.* 

Converging  rays  which  are 
reflected,  have  the  same  con- 
vergency  at  an  equal  distance 
as  if  they  passed  in  straight  lines. 

Thus;  as  the  converging  rays  i  n  and  k  o  have 
their  angles  of  incidence  equal 
to  their  angles  of  reflection,  they 
converge  to  the  point  m,  at  a 
distance  equal  to  l,  where  the 
rays  would  have  met  in  an  un- 
interrupted course,  t 

By  spherical  or  convex  sur- 
faces parallel  rays  are  rendered 
divergent.   Every  spherical  sur- 
face may  be  considered  as  composed  of  an  infinite 
number  of  straight  lines. 

Suppose  E  F  two  of  those  straight  lines  which  form 
part  of  the  convex  surface 
of  a  sphere,  and  that  the 
rays  a  f,  b  e,  fall  parallel 
on  the  points  e  and  f,  thenJD 
it  is  evident  that,  as  these 
lines  are  reflected  in  equal 
angles  from  the  oblique  lines 
E  and  F,  they  will  diverge 
and  lose  their  parallel  direction. 

*  The  arc  e  f  will  equal  the  arc  c  d,  only  when  the  right  line, 

»r  reflecting  surface  g  h,  passes  through  the  centre  of  the  circle. 

t  Supposing  o  N  to  pass  through  the  centre  of  the  circte.  Ed. 


Reflected  Vision .  181 

By  considering  converging  rays  in  the  same  man- 
ner it  appears  that  they  will  become  less  conver- 
gent, and  diverging  rays  more  divergent. 

In  concave  surfaces  parallel  rays  are  made  con- 
vergent. 

For,  as  this  surface,  like  that  of  the  convex  mirror, 
may  be  supposed  to  be  formed  by  an  infinite  num- 
ber of  right  lines,  the  parallel  rays  k  h 
and  I  G  which  fall  on  the  oblique  lines 
G  and  H  are  reflected  in  equal  angles 
through  the  lines  g  l  and  h  m,  tending 
towards  each  other  till  they  meet  in 
the  vertex. 

In  like  manner  it  may  be  shown,  on 
the  same  superficies,  that  rays  already 
convergent  become  still  more  so,  and  that  diverging 
rays  are  made  less  divergent. 

From  the  foregoing  principles  it  will  be  easy  to 
comprehend  the  effects  of  mirrors,  and  account  for 
the  principal  phenomena  which  may  occur,  either 
with  those  that  are  plane,  convex,  or  concave. 

A  plane  mirror  does  not  alter  the  figure  or  change 
the  size  of  objects;  but  the  whole  image  is  equal  and 
similar  to  the  whole  object,  and  has  the  same  situation 
on  one  side  of  the  plane  that  the  object  has  on  the 
other. 

A  spectator  will  see  his  own  image  as  far  beyond  a 
mirror  as  he  is  before  it,  and  as  he  moves  to  or  from 
the  mirror,  the  image  will  at  the  same  time  advance 
to,  or  recede  from  him,  on  the  opposite  side,  but 
seemingly,  with  double  velocity,  because  the  two 
motions  are  equal  and  contrary. 

If  a  person  view  himself  in  a  plane  looking-glass, 
he  will  see  himself  completely,  at  any  distance,  in  a 
part  of  the  glass  the  length  and  breadth  of  which  are 
equal  to  half  the  length  and  breadth  of  the  correspon- 
ding parts  of  his  body ;  for,  as  the  image  appears  as 
far  behind  the  glass  as  he  stands  before  it,  the  part 

Y 


182  Lenses. 

of  the  mirror  on  which  the  rays  fall  will  be  equal  to 
half  the  length  or  breadth  of  the  object,  or  the  rays 
will  only  spread  half  as  much  as  they  wou.d  do  at 
double  the  distance. 

In  a  convex  mirror  the  image  always  appears  smal- 
ler than  the  object,  and  the  diminution  increases  as 
the  object  recedes.  This  will  be  easily  understood, 
when  it  is  considered  that  the  reflecting  convex  sur- 
face of  a  mirror  renders  incident  converging  rays  less 
convergent. 

The  image  does  not  appear  so  far  behind  a  reflect- 
ing convex  mirror  as  in  a  plane  one ;  for  the  diverging 
rays  are  reflected  more  divergent,  consequently  they 
have  their  imaginary  focus  much  nearer,  which 
makes  the  image  appear  nearer  to  the  surface  of  re- 
flection. 

A  concave  mirror  differs  from  the  two  preceding. 
It  only  shows  bodies  erect  when  the  object  is  placed 
between  its  real  focus  and  the  mirror;  then  the  rays 
are  rendered  convergent,  and  the  image  appears 
larger  than  the  object;  but  when  it  is  placed  beyond 
the  focus  of  the  mirror,  the  rays  cross  each  other  at 
the  focus,  and  the  image  appears  inverted. 

Lenses, 

A  L  E  N  s  is  a  piece  of  glass  or  crystal  so  formed, 
that  rays  of  light  in  passing  through  it  have  their  di- 
rection changed;  it  either  converges  them  to  a  point 
or  focus  beyond  the  lens,  or  diverges  them  as  if  the 
rays  had  proceeded  from  a  point  before  it,  or  brings 
converging  and  diverging  rays  parallel  to  each  other. 

The  lens  marked  >      ^ 

1,  is  called  plano^         a       ^       jL     Ji^       cT^ 
convex^  having  one  A 

of  its  sides  plane, 

and  the  other  sphe- 
rical, which  forms 


Lenses* 


183 


the  segment  of  a  sphere.  2.  Is  double  convexy 
having  both  sides  the  same,  and  is  like  two  equal  seg- 
ments of  a  sphere  joined  together.  3.  Is  a  plano-coit- 
cave  lens;  one  of  its  sides  being  flat,  and  the  other 
hollow,  such  as  would  be  represented  by  the  impres- 
sion of  a  small  part  of  a  sphere  in  soft  wax.  4.  Is 
double  concave,  having  both  its  sides  equally  hollow. 
The  5th  is  called  a  meniscus,  having  one  of  its  sides 
concave,  and  the  other  convex. 

The  line  b  d,  which  passes  through  the  middle  of 
the  lens  perpendicular  to  the  sides,  is  called  the  axis; 
the  two  points  where  it  enters  and  passes  out  of  the 
lens,  the  vertices;  and  the  distance  between  them, 
the  diameter.  The  focus,  either  of  converging  or, 
diverging  rays,  is  situated  somewhere  in  the  axis  of 
the  lens. 

If  the  ray  of  light  a  e  fall  upon  the  plano-convex 

F 


lens  at  e,  it  will  not 
pass  on  in  tHe    right 


line  A  H ;  but  as  it  is 
transmitted  through 
the  glass  its  course 
will  be  refracted  into 
the  line  E  d,  approach- 
ing towards  the  per- 
pendicular of  the  convex  side  f  g.  Another  ray  fall- 
ing on  c  parallel  to  a  e,  and  equidistant  from  the  axis 
B,  will  be  refracted  in  like  manner,  and  will  converge 
and  meet  a  e  in  d,  the  focal  point  of  the  lens.  All  the 
intermediate  rays  which  fall  between  e  and  c  con- 
verge in  the  same  way,  only  the  rays  will  be  less  re- 
fracted as  they  approach  towards  b,  till  they  fall  into 
the  centre  and  p^ss  in  a  right  line  through  the  diame- 
ter to  the  converging  point  d. 


184 


Lenses. 


A- 


F 

c 

::>& 

^eS 

— ^I 

When  parallel  rays  fall  on  the  flat  surface  of  the  same 
figure,  they  ,  will  tend 
from  the  perpendicular 
F  G  of  the  convex  side, 
and  converge  with  tlic 
pencil  of  rays  from  i  £ 
and  the  intermediate  pa- 
rallel rays  to  the  point  a. 

Considering  the  two 
mediums,  air  and  glass,  through  which  the  rays  pass, 
it  appears  that  a  ray  from  the  spherical  surface  of  the 
lens,  whether  in  passing  through  the  denser  medium  of 
the  glass,  or  the  rarer  medium  of  the  air,  still  converges 
.to  a  point  somewhere  in  the  axis.  Therefore  if  both 
sides  of  the  lens  be  spherical,  the  convergency  will  still 
be  greater,  and  the  rays  will  meet  in  some  point  nearer 
to  the  centre  of  the  lens. 

Consequently  if  a  b  be 
a  double  convex  lens,  the 
parallel  rays  which  fall 
upon  it  will  converge  to 
the  point  c ,  by  the  double 
convexity  of  its  sides ; 
whereas,  if  one  of  its  sides 
had  been  flat,  as  in  the  preceding  example,  the  con- 
verging point  would  have  been  extended  to  d. 

If  A  B  be  a  plano-concave  lens,  and  c  d  the  axis;  let 
the  ray  a  g  fall  upon  the 


lens  at  a,  then  it  will 
be  refracted  iy  passing 
through  the  glass,  and 
diverge  from  the  direct 
line  G  I  into  a  d,  ap- 
proaching towards  the 
perpendicular  of  the  con- 
cave side.  The  ray  at  b  being  equidistant  from  c  will 
diverge  in  the  same  manner,  as  well  as  the  rest  of  the 


Lenses, 


185 


D-^gvii^i^^^^^^ 


intermediate  rays,  in  proportion  to  their  distance  from 
the  axis  of  the  lens. 

Let  the  rays  fall  on  the  flat  side  of  the  lens:  after 
having    passed    through  ^ 

the  denser  medium  of  the  C 

•  glass,  they  will  diverge 
on  the  opposite  side  as 
they  pass  into  the  rarer 
medium  of  the  air,  and 
the  ray  g  a  will  be  refract- 
ed from  the  perpendicular 
F  E,  as  if  it  proceeded  from  d;  likewise  c,  and  all 
the  mtermediate  rays,  will  diverge  in  proportion  to 
their  distance  from  the  axis. 

Consequently,  if  the  rays  equally  diverge  from  the 
hollow  surface,  either  in  passing  from  or  into  the  lens 
there  will  be  an  equal  divergency  from  both  sides  of 
the  double  concave  lens  a  b  ;   and  those  rays,   which 
would   have    di- 
verged in  a  pla- 
no-concave   lens 
as  coming  from 
c,^  now   diverge  C^«jS)Vg::::: 
with  the  addition- 
al concave  side, 
as  if  they  issued 
from  the  virtual 
focusEjinthe  ax- 
is of  the  lens  c  D. 

When  the  radiant,  or  object,  is  at  a  considerable  dis- 
tance from  the  lens,  the  rays  issuing  from  it  will  fall 
upon  the  glass  and  converge"  to  the  focal  point;  whence 
the  image  will  appear  inverted,  but  clear  and  distinct, 
as  if  the  object  was  placed  there  in  the  same  position. 
The  image  removes  further  from  the  lens  as  the  radiant 
approaches;  so  that  when  it  is  brought  within  the  focal 
point,  the  rays  diverge  to  infinite' distances,  passing 
through  the  lens  in  parallel  lines. 


186  Lenses. 

If  A  B  represent  a  double  convex  lens,  and  c  d  an 
object  at  a  considerable  distance  from  the  glass,  the 


v;s 

c.. :::■ — 

r-~ 

I— :sjt»*t:'.„ — — 


rays  d  a  and  d  b  issuing  from  the  point  d,  will  be  re- 
fracted at  A  and  b,  and  converge  to  e,  and  form  one  ^ 
extremity  of  the  object ;  in  the  same  manner  c  a  and 
c  b,  in  passing  from  c,  will  converge  to  f,  the  other 
extremity ;  then  if  we  conceive  an  infinite  number  of 
rays  passing  from  every  other  point  of  the  object  c  d, 
they  will  cross  each  other  and  meet  somewhere  be- 
tween E  and  F,  the  foci  of  the  lens,  forming  a  complete 
image  of  the  object  reversed,  and  the  linear  magnitude 
of  the  object  and  image  will  be  relatively  as  their  dis- 
tance from  the  lens. 

Camera  Obscura. 

This  machine  is  formed  on  the  above  principle; 
for,  if  G  H  represent  a  darkened  room,  and  a  b  a  lens 
fixed  in  the  side  of  it,  the  object  c  d,  with  all  its  sha- 
dows and  colours,  will  be  distinctly  seen  on  a  white 
surface  placed  in  the  focus  of  the  glass,  but  in  an  in- 
verted position.  As  the  appearance  of  inverted  objects 
is  unpleasant  to  the  eye,  if  the  lens  of  the  camera  be 
placed  in  a  short  tube  on  the  top  of  a  small  build- 
ing, and  the  image  of  the  objects  be  reflected  through 
the  lens  by  an  inverted  mirror  placed  above  it,  the  pic- 
ture will  be  presented  in  a  proper  position  upon  die 
receiving  table  in  the  focal  point  of  the  lens ;  giving 
the  most  beautiful  and  animated  representation  of  all 
the  surrounding  objects  in  their  own  colours. 


187 


'  >  0  ■". 


The  Magic  Lantern. 

This  amusing  machine  is  made  to  magnify  small 
pictures,  which  are  pauited  upon  glass,  and  to  throw 
the  shadow  upon  the  side  of  a  darkened  room;  it  is 
principally  formed  by  a  convex  lens. 

A  lighted  candle  or  lamp  is  placed  in  the  inside  of  a 
square  box,  which  has  the  tube  g  b  projecting  from 


its  side;  g  h  is  a  thick  plano-convex  lens,  which 
strongly  illuminates  the  object  e  f  when  it  is  put  in- 
verted  mto  the  tube;  k  is  a  concave  reflecting  mirror 
to  give  additional  force  to  the  light;  and  a  b  is  a  dou- 
ble convex  lens,  placed  in  a  moveable  tube,  which 
slides  m  the  interior  of  the  projecting  tube  g  b  :  when 
this  lens  is  properly  adjusted  it  throws  the  shadow  of 
the  object  large  and  upright  against  the  side  of  the 
wall.  The  magnitude  of  the  shadow  c  d  is  represented 
as  much  larger  than  the  image  e  f,  as  the  distance  c  a 
IS  greater  than  e  a. 

Burning  Glass. 

This   is  a  double  convex,   or  plano-convex  lens, 
which  collects  the  sun's  rays  upon  its  surface,  and  con- 
verges them  into  a  point  called  the  focus:   when  the 
rays  are  thus  concentrated  they  burn  with  great  ardour 
and  will  melt  the  densest  metals.  ' 


188  Telescopes. 

As  all  those  rays  which  fall  upon  the  surface  of  a 
lens  are  collected  in  its  focus,  the  effect  will  be  in  pro- 
portion to  the  difterence  between  the  surface  of  the 
lens  and  the  surface  of  the  focus;  therefore,  if  a  lens 
four  inches  in  diameter  collect  the  sun's  rays  at  the 
distance  of  a  foot  from  the  glass,  the  image  at  the  focus 
will  not  be  more  than  one  tenth  of  an  inch  broad,  so 
that  the  surface  is  more  than  sixteen  hundred  times 
less  than  that  of  the  glass;  therefore  the  sun's  rays  are 
so  many  times  more  dense  at  that  point  than  on  the 
surface.  Burning  glasses  have  been  made  three  feet  in 
diameter,  and  the  rays,  after  passing  through  them, 
have  been  collected  again  by  another  lens  placed  pa- 
rallel to  the  former,  so  as  to  converge  them  into  a  still 
smaller  point  at  the  focus.  By  the  intense  heat  of  the 
rays  thus  combined,  gold  has  been  fluxed  in  a  few  se- 
conds, and  sheet  iron  melted  in  a  moment. 

Telescopes, 

A  TELESCOPE  is  an  optical  instrument  for  discover- 
ing those  distant  objects  that  are  invisible  to  the  naked 
eye,  or  for  rendering  more  clear  and  distinct  those  that 
are  discernible ;  it  is  constructed  to  act  either  by  re- 
fraction or  reflection. 

No  invention  in  the  mechanic  arts  has  ever  proved 
more  useful  and  entertaining  than  the  production  of  the 
telescope ;  its  utility  both  by  sea  and  land  is  too  well 
known  to  need  observation.  With  respect  to  the  know- 
ledge of  the  heavenly  bodies,  we  owe  much  to  the  in- 
vention of  the  telescope,  for  without  such  assistance 
the  bcience  of  astronomy  must  have  been  far  short  of 
its  present  state. 

The  first  invention  is  attributed  to  John  Baptista 
Porta,  a  Neapolitan,  about  two  centuries  and  a  half  ago; 
but  Galileo  soon  afterwards  greatly  improved  it,  and 
by  this  means  added  considerably  to  the  catalogue  of 
fixed  stars.    Galileo's  telescope  passed  on  for  many 


Astronomical  Telescope,  189 

years  without  material  alteration,  till  Gregory  and  New- 
ton undertook  the  construction  of  telescopes,  and 
brought  them  to  a  considerable  degree  of  perfection, 
which  has  been  completed  by  Herschel  and  others  in 
the  present  day. 

'  There  are  many  kinds  of  telescopes ;  but  as  it  would 
greatly  exceed  our  plan  to  enter  into  a  description  of 
them  all,  it  will  be  sufficient  to  describe  some  of  the 
most  material,  such  as  the  Astronomical  Telescope, 
the  Day  or  Land  Telescope,  the  Newtonian  and  Gre- 
gorian. First,  let  it  be  premised,  that  the  object-glass 
is  that  lens  which  is  placed  at  the  end  of  the  tube  near- 
est the  object;  the  eye-glass  is  that  which  is  nearest  the 
the  eye,  and  when  there  are  more  lenses  than  one  in  the 
tube,  beside  the  object-glass,  they  are  called  eye-glasses 
likewise. 

The  Astronoinical  Telescope, 

This  consists  of  an  object  and  eye-glass  fitted  into 
a  long  tube.  The  object  glass,  which  is  a  segment  of 
a  large  sphere,  is  made  either  double  convex  or  plano- 
convex; the  eye-glass  is  double  convex,  formed  from  a 
segment  of  a  small  sphere,  and  these  glasses  are  placed 
in  the  tube  at  the  common  distance  of  their  foci. 

Suppose  rays  of  light  issuing  from  every  part  of  the 
object  H  I,  fall  upon  the  object-glass  a  b  ;  in  passing 

H  A 

y^v.«^r...... A 


I_. — 


through  it  they  will  be  refracted  and  converged  into  tlie 
foci  E  K,  where  the  inverted  image  of  the  object  will  be 
formed;  then,  if  the  eye-glass  c  d,  which  is  of  shorter 

Z 


190  La)id  Telescope* 

focal  distance,  be  so  placed  as  to  include  e  k,  the  rays 
will  pass  on  through  c  d,  in  a  position  nearly  parallel, 
cross  each  other  at  f,  and  form  a  large  but  inverted 
image  of  the  object  on  the  retina  at  c.  The  objects  will 
be  magnified  by  this  glass  in  proportion  as  the  distance 
of  the  focus  of  the  object-glass  m  e,  exceeds  the  dis- 
tance of  the  focus  of  the  eye-glass  e  l. 

Land  Telescope. 

This  instrument  is  used  for  viewing  objects  in  the 
day  time,  on  the  surface  of  the  earth,  it  is  usually 
formed  by  three  double  convex  eye-glasses,  and  a  dou- 
ble convex  or  plano-convex  object-glass;  it  exhibits  the 
objects  in  an  upright  position,  and  the  lenses  are  dis- 
posed in  such  a  manner  in  the  tube,  that  the  distance 
between  any  two  may  be  the  aggregate  of  the  distance 
of  their  foci;  so  that  an  eye  placed  in  the  focus  of  the 
first  glass  will  see  objects  upright  and  distinct,  and 
magnified  in  the  ratio  of  the  distance  of  the  focus  of  the 
object-glass  to  the  distance  of  the  focus  of  the  eye-glass 
at  the  opposite  extremity. 

If  A  B  be  the  object,  the  rays  from  which  are  received 
by  the  object-glass  c  d,  they  enter  the  first  eye-glass 


G  H,  but  instead  of  falling  into  the  eye,  as  in  the  astro- 
nomical telescope,  they  pass  on  to  i  k,  another  lens 
equally  convex,  which  is  placed  at  double  its  focal  dis- 
tance from  G  H,  so  that  the  rays  are  transmitted  parallel 
through  the  interval  between  them,  and  cross  each  other 
in  the  common  focus  of  g  h  and  i  k.  After  passing 
IK,  the  rays  are  again  converged  into  the  foci  l  m, 
where  the  image  is  formed  in  a  position  the  reverse  of 


Rejiecting  Telescopes,  191 

E  f;  these  rays  are  again  transmitted  through  n  o,  and 
are  tliea  collected  on  the  retina  of  the  eye  at  p,  where 
the  image  is  clearly  formed,  and  in  an  upright  position. 

As  tiie  addition  of  glasses  in  the  land  telescope  does 
not  magnify  the  object,  an  astronomical  telescope  may 
be  used  as  a  land  telescope,  by  having  an  extra  tube 
with  eye-glasses  made  to  slide  into  the  end  of  the  tele- 
scope; or  that  which  is  used  in  the  day  may  be  used  for 
astronomical  observations,  by  taking  out  two  of  the 
glasses. 

Land  telescopes  are  sometimes  made  with  three 
glasses  only,  and  some  with  five ;  but  the  dimness  of 
the  latter  is  equally  inconvenient  with  the  false  repre- 
sentation of  the  former;  so  that  four  glasses,  the  me- 
dium, appears  the  best  calculated  to  avoid  the  imper- 
fections of  either. 

The  aberration  and  colouring  in  the  rays  of  light,  as 
they  are  transmitted  through  lenses,  was  a  great  obsta- 
cle to  the  improvement  of  telescopes,  till  a  late  optician, 
DoUond,  contrived  to  form  the  lenses  of  different  kinds 
of  glass,  which  mutually  correct  each  other's  refrangi- 
bility,  and  greatly  remedy  the  defects.  An  instrument 
thus  fitted  up  is  called  an  Achromatic  Telescope. 

Rejiecting  Telescopes, 

Reflecting  telescopes  are  those  which  are  chiefly 
formed  by  mirrors,  and  reflect  the  object  to  the  eye  in- 
stead of  refracting.  They  are  principally  confined  to 
two  kinds,  called  the  Gregorian  and  Newtonian. 


192  Meflecti?ig  Telescopes* 

In  the  construction  of  the  JVeivtonian,  let  a  b  c  d  be 

C  EA 

I 
H 


TB 


a  large  tube,  open  at  the  end  c  d,  and  closed  at  a  b;' 
the  length  b  d  being,  at  least,  equal  to  the  focus  of  the 
metallic  reflector  f  e,  which  is  placed  near  the  end  of 
the  tube.  The  rays  g  ^,  i  i,  that  come  from  h,  a  distant 
object,  being  considered  as  parallel  to  each  other,  fall 
on  the  concave  speculum  e  f,  and  are  reflected  back 
upon  a  small  plane  speculum  l,  which  is  inclined  in 
an  angle  of  45*^;  from  this  mirror  they  are  again  re- 
flected to  a  convex  lens,  which  is  fixed  in  the  side  of 
the  tube,  and  converged  into  n  ,  the  focus  of  the  glass, 
where  the  image  appears  magnified  and  distinct  to  the 
eye.  By  fixing  an  additional  tube,  with  lenses  to  the 
side  of  the  telescope  al  n,  the  object  may  be  either  in- 
creased farther  or  diminished,  and  brought  into  an  up- 
right position. 

The  Gregorian  telescope  a  b  c  d,  is  a  large  brass 


Al?- 


HQI 


tube,  in  which  is  placed  a  concave  metallic  speculum 
E  E,  with  a  round  hole,  ee,  perforated  in  the  middle; 
F  G  is  a  small  concave  mirror  fastened  to  the  rod  k, 
which  is  moved  back\vards  or  forwards  at  pleasure.  If 
L  represent  an  object  at  a  considerable  distance,  and 


Reflecting  Telescopes.  193 

its  rays  o  o,  p  /?,  enter  the  tube  parallel  to  each  other, 
they  will  fall  on  the  larger  speculum,  e  e;  from  which 
they  are  reflected  into  the  foci  at  m  ,  where  an  inverted 
image  is  formed;  but  after  crossing  each  other,  the  rays 
fall  on  the  concave  speculum  f  g,  the  centre  of  which 
e  is  the  axis  of  the  tube  z  e.  From  f  g  the  rays  would 
again  converge  into  the  foci  q^q^  with  image  upright; 
but  in  converging  to  bring  them  close  to  the  eye,  they 
fall  upon  a  convex  lens  at  s  s,  by  which  the  image  is 
formed  in  the  focus  betv^'een  i  i,  and  thence  taken  up 
and  carried  to  the  eye  at  jz  by  a  meniscus  h,  where  the 
image  appears  magnified,  upright,  clear,  and  distinct. 

The  latter  of  these  reflecting  telescopes  is  now  gene- 
rally used,  as  it  shows  all  objects  in  their  natural  posi- 
tions, and  is  of  a  form  the  most  convenient  for  portabi- 
lity and  readiness  in  management. 


194 


SOLAR  SYSTEM, 

The  power  of  God  being  made  manifest  even  in 
the  smallest  of  his  works,  how  much  must  the  human 
mind  be  led  to  contemplate  his  infinite  wisdom  and 
power,  when  we  survey  the  multitude  of  stars  that  are 
scattered  through  the  infinity  of  space!  Judging  from 
analogy  of  the  general  purposes  of  creation,  we  can 
hardly  conceive  them  to  be  placed  for  mere  ornament, 
or  even  for  the  purpose  of  giving  light  in  the  absence 
of  the  sun,  when  the  reflection  of  his  rays  from  the 
moon,  a  single  satellite,  gives  a  thousand  times  more 
light  to  the  earth  than  the  whole  of  the  stars.  Rather 
let  it  be  presumed,  that  the  great  number  of  stars, 
which  have  no  revolutionary  motion,  are  destined  for 
more  important  purposes;  and,  like  the  sun  in  our 
system,  are  inexhaustible  fountains  of  light  and  heat, 
which  diffuse  their  vivifying  powers  to  their  own  sur- 
rounding orbs ;  the  opacity  of  whose  bodies,  and  the 
immensity  of  their  distance  from  us,  render  them  in- 
visible to  our  eyes.  Thus  the  extent  of  imagination 
falls  infinitely  short  in  comprehending  the  greatness 
of  God  or  his  works.  We  form  a  comparative  idea  of 
the  distance  of  a  mile,  a  thousand  or  a  million  of  miles ; 
but  where  are  the  bounds  of  that  almighty  power, 
which  .created  those  innumerable  systems  that  are 
scattered  at  such  immense  distances  from  each  other 
through  the  infinity  of  space? 

The  sun,  and  the  planetary  worlds  which  revolve 
round  it,  is  one  of  those  numerous  systems  that  we 
are  now  about  to  consider. 

Various  opinions  have  been  adopted,  at  different 
times,  with  respect  to  the  motion  of  the  sun  and  pla- 
nets; but  as  the  Copernican  system  is  now  establish- 


Solar  System'  195 

ed,  and  accepted  by  all  enlightened  nations  as  the  most 
consonant  to  reason  and  the  operations  of  nature,  it 
will  be  sufficient  for  our  purpose,  before  we  enter  into 
an  explanation  of  it,  merely  to  mention  two  others, 
which  at  different  times  have  had  their  disciples. 

The  first  is  called  the  Ptolemean,  from  Ptolemy 
its  framer,  who  was  born  at  Pelusium,  in  Egypt,  and 
flourished  as  a  great  mathematician  and  astronomer 
soon  after  the  commencement  of  the  Christian  sera. 

Guided  by  the  sensible  appearances  of  the  heavenly 
bodies,  without  considering  either  their  absolute  or 
relative  motion,  he  considered  the  earth  as  a  stationary 
body,  fixed  in  the  centre  of  the  system,  and  that  the 
sun  and  planets  were  subordinate,  and  revolved  round 
the  earth  in  twenty-four  hours. 

After  the  Ptolemean  the  other  was  formed  by  Tycho 
Brahe,  a  Dane;  who  considered  the  earth  to  be  placed 
in  the  centre  of  the  universe,  and  that  the  sun  revolv- 
ed round  it,  whilst  the  rest  of  the  planets  revolved 
round  the  sun.  In  an  improvement  of  this  system,  a 
diurnal  motion  was  given  to  the  earth  round*  its  own 
axis  to  account  for  day  and  night,  more  naturally  than 
by  a  revolution  of  the  whole  system  in  twenty-four 
hours.    But  this  comphcated  and  ill-digested  hypo- 
thesis soon  fell  into  disrepute,  to  make  way  for  that 
called  the  Copernican  system :  which  is  long  likely  to 
endure,  as  a  monument  of  human  ingenuity,  and  a 
rational  system  of  the  planetary  motions. 
^  Nicholas  Copernicus  was  born  at  Thorn,  in  Prus- 
sia, in  the  year  1473.  He  rather  revised  and  perfected 
the  doctrine  of  Pythagoras,  who  existed  about  600 
years  before  Christ,  than  created  any  new  system  of 
his  own.  The  Pythagorean  idea  of  the  universe  at- 
tracted the  mind  of  Copernicus,  and,  after  a  labour  of 
twenty  years,  he  brought  it  to  perfection,  and  died 
just  in  time  to  save  himself  from  the  bigoted  persecu- 
tion of  the  Romish  church  for  his  discovery. 


196 

The  Copernican^  or  Solar  System, 

This  supposes  the  sun  placed  in  the  centre  of  our 
system,  and  that  the  earth  and  the  other  planets,  re- 
volve round  it  in  different  orbits  at  immense  distances 
from  each  other. 

These  planets,  which  we  perceive  by  the  reflection 
of  the  sun's  rays  from  their  opake  bodies,  are  of  three 
kinds,  called  primary,  secondary,  and  comets. 

The  primary  planets  are  those  which  move  round 
the  sun  in  orbits  nearly  circular  and  concentric,  but 
at  different  distances  from  it;  they  are  seven  in  num- 
ber, called  ^  Mercury,  9  Venus,  ©  the  Earth,  o  Mars, 
%  Jupiter,  I2  Saturn,  and  j^  the  Georgium  Sidus. 
Mars,  Jupiter,  Saturn,  and  the  Georgium  Sidus,  are 
usually  called  superior  planets,  because  their  orbits 
include  that  of  the  earth.  Venus  and  Mercury  are 
called  inferior  planets,  as  their  orbits  are  contained 
within  the  earth's  orbit. 

In  addition  to  these,  are  three  or  four  others  which 
have  been  lately  discove  ed,  but  their  magnitude  and 
revolutions  have  not  yet  been  correctly  defined. 

The  secondary  planets^  which  are  likewise  called 
satellites  or  moons,  are  attendants  on  primary  planets, 
and  revolve  round  them  whilst  the  primary  planets  cir- 
cle round  the  sun.  The  earth  is  accompanied  by  one 
moon,  Jupiter  by  four,  Saturn  by  seven,  and  the  Geor^ 
gium  Sidus  by  six. 

The  comets  are  bodies  which  revolve  round  the  sun 
in  the  planetary  region,  but  their  number  and  periodical 
revolutions  have  not  yet  been  correctly  detennined; 
they  move  in  very  eccentric  orbits,  suddenly  appear 
and  disappear,  and  are  usually  attended  by  a  long  train 
of  light,  which  is  called  the  tail  of  the  comet. 

The  adjoining  diagram  is  the  usual  representation  of 
the  solar  system  in  piano,  but  it  is  not  strictly  correct, 
as  the  planets  move  in  elliptical  orbits,  die  planes  of 
which  do  not  exactlv  coincide  with  one  another. 


Solar  System. 


197 


o  represents  the  sun  placed  in  the  centre  of  the  sys- 
tem, M  1  is  the  orbit  of  Mercury  round  the  sun,  v  2 
the  path  of  Venus,  e  3  the  orbit  of  the  earth,  m  4  the 
orbit  of  Mars,  j  5  the  course  of  Jupiter,  s  6  that  of  Sa- 
turn,  and  c  s  7  is  the  orbit  of  the  Georgium  Sidus. 

All  the  planets  are  supposed  to  have  a  compound, 
motion,  like  the  earth,  called  the  annual  and  diurnal. 

The  annual  motion  is  that  with  which  they  pass 
through  their  orbits  from  west  to  east,  forming  a  year 
by  one  complete  revolution  round  the  sun. 

The  diurnal  motion  is  the  rotation  of  a  planet  round 
an  imaginary  line  passing  through  its  centre,  called  its 
axis,  whilst  it  is  moving  through  its  annual  orbit ;  one 
complete  rotation  is  called  a  day.  This  compound  pla- 
netary  motion  may  be  conceived  by  the  rolling  of  a  ball 

2A 


198  Solar  System. 

upon  a  table,  which  is  perpetually  turning  round  whilst 
it  passes  from  one  extremity  to  the  other. 

The  sun,  moon,  planets,  and  fixed  stars,  all  appear 
to  be  placed  in  the  same  concave  sphere,  of  which  the 
eye  of  the  spectator  seems  to  be  the  centre ;  so  that  the 
bodies  apparently  differ  in  magnitude,  but  not  in  their 
distances.  We  estimate  the  distance  of  objects  on  the 
surface  of  the  earth  by  some  given  measure  or  compa- 
rative proportion  with  some  other  objects  less  remote; 
but  in  viewing  celestial  bodies,  the  immensity  of  their 
distance  affords  us  no  relative  means  of  forming  our 
judgment  of  their  respective  positions.  Therefore  the 
optical  sense  is  deceived;  for  demonstrations  show  us 
that  the  sun  is  nearer  to  us  than  the  fixed  stars,  the  ocu- 
lar proof  from  eclipses  convinces  us  that  the  moon  is 
nearer  the  earth  than  the  sun,  and  our  i^eason  teaches 
us  to  believe  that  some  of  the  stars  are  many  millions 
of  niiles  nearer  to  us  than  others.  But  an  easy  experi- 
ment will  show  how  unable  we  are  to  judge  of  distances 
by  our  sight  alone;  for,  in  a  dark  night,  ijf  afew  lighted 
candles  be  placed  at  different  distances  from  a  spectator, 
they  will  all  appear  equally  remote,  but  the  flames  will 
vary  in  magnitude  according  to  the  distance,  and  the 
person  will  be  unable  to  judge,  with  any  correctness, 
how  far  he  is  from  them. 

The  planets  in  their  annual  courses  cross  the  eclip- 
tic, or  sun's  apparent  path,  in  two  opposite  points  called 
the  nodes;  but  the  planets  do  not  move  in  the  same 
plane  with  each  other,  as  they  cross  the  ecliptic  in  dif- 
ferent parts  of  the  heavens :  this  may  be  properly  repre- 
sented by  placing  different  sized  hoops  within  each 
other  in  different  directions;  considering  the  centre  as 
the  sun's  place,  and  the  hoops  themselves  as  the  orbits 
of  the  planets. 

It  has  been  already  observed,  that  the  orbits  of  the 
planets  are  not  accurately  represented  by  the  circum- 
ference of  a  circle,  for  their  course  is  elliptical,  with 
clitferent  eccentricities. 


Solar  System.  '  199 

A  E  B  F  represents  the  orbit 
of  a  planet  with  the  sun  in  one 
of  the  foci  c  d;  the  axis  a  b  is 
called  the  line  of  the  apsides; 
and  when  the  sun  is  at  d,  and 
the  planet  at  a,  its  greatest  dis- 
tance from  it,  the  planet  is  said 
to  be  in  its  aphelion,  or  higher 

apsis;  when  it  is  at  the  other  extremity  b,  or  nearest 
the  sun,  it  is  then  in  its  perihelion,  or  lower  apsis. 

The  mean  distance  of  a  planet  from  the  sun  is, 
when  the  planet  is  in  either  extremity  of  the  conju- 
gate diameter  e  f. 

Two  planets  are  said  to  be  in  conjunction  when 
they  both  appear  equally  advanced  in  the  same  part 
of  the  heavens ;  and  when  they  are  in  opposite  points 
they  are  said  to  be  in  opposition. 

Each  planet  has  its  peculiar  course,  which  it  al- 
ways pursues  without  deviation ;  the  whole  courses 
of  the  planets  are''  included  in  a  certain  zone  or  belt 
of  the  heavens,  extending  between  18^  and  19^  in 
breadth,  which  is  called  the  Zodiac,  containing  the 
constellations  Aries,  Taurus,  Gemini,  Cancer,  Leo, 
Virgo,  Libra,  Scorpio,  Sagittarius,  Capricornus, 
Aquarius,  and  Pisces. 


200 


TJie  Sun  and  Planets. 

The  Sun  is  the  great  luminary  which  dispenses 
light  and  heat  to  all  the  planetary  system.  It  has 
been  usually  reckoned  amongst  the  planets,  but  it 
more  properly  belongs  to  the  fixed  stars,  as  one  of 
those  central  bodies  dispersed  through  the  infinity  of 
space  which  have  their  subordinate  orbs  revolving 
round  them.  The  sun  is  placed  nearly  in  the  centre 
of  our  system,  and  revolves  round  its  own  axis  in 
25i  days;  the  axis  has  an  inclination  of  about  eight 
degrees  with  the  ecliptic. 

Although  its  apparent  diameter  is  seen  from  the 
^arth  under  an  angle  of  32'  \2"  only,  the  real  diame- 
ter of  this  beautiful  luminary  is  not  less  than  890 
thousand  miles,  and  it  is  about  1392500  times  bigger 
than  our  earth,  which  is  near  96  millions  of  miles 
distant  from  it.  It  appears  to  us  tohave  a  revolving 
motion  through  an  orbit  from  east  to  west;  but  this 
apparent  motion  will  be  hereafter  shown  to  arise  from 
the  diurnal  motion  of  the  earth  from  west  to  east, 
whilst  it  is  passing  through  its  annual  orbit.  When 
the  sun  is  viewed  through  a  telescope  it  seems  to 
have  dark  spots  on  its  disk,  which,  from  its  globular 
form  and  revolving  motion,  alter  their  shape  and  oc- 
casionally disappear:  the  various  opinions  that  are 
given  with  respect  to  these  spots  leave  us  still  in  con- 
siderable doubts. 

Mercury  is  the  least  of  all  the  planets  and  nearest 
the  sun,  which  makes  it  seld(>m  visible  to  us,  as  its 
reflected  light  is  absorbed  in  the  sun's  more  powerful 
rays.  Its  greatest  elongation  or  distance  from  the  sun, 
as  viewed  from  the  earth,  is  not  more  than  28^;  it  is 
computed  to  be  about  thirty-seven  millions  of  miles 
distant  from  the  sun,  and  revolves  round  its  orbit  in 
^7  d.  23  h.  which  forms  its  year.    The  diameter  of 


The  Sun  and  Planets^  201 

Mercury  is  three  thousand  miles ;  it  contains 
28274000  squ&re  miles  on  its  surface,  and  moves  at 
the  rate  of  110680  miles  in  an  hour.  When  it  is  seen 
through  a  telescope,  its  edge  appears  clear  and  dis- 
tinct. Its  body  is  opake,  and,  like  the  moon,  reflects 
a  borrowed  light,  and  changes  its  phases  or  appear- 
ance, according  to  its  several  positions.  When  it 
passes  over  the  sun's  face,  or  is  between  us  and  the 
sun,  this  is  called  its  transit,  and  the  planet  appears 
like  a  black  spot  in  the  sun's  disk. 

Femis  has  generally  a  larger  and  brighter  appear- 
ance than  any  other  planet,  which  makes  it  easily  dis- 
tinguishable from  the  rest. 

Its  diameter  is  7699  miles,  and  its  distance  is 
69500000  miles  from  the  sun ;  it  revolves  through 
its  orbit,  or  completes  its  year  in  224  d.  6  h.  and 
moves  at  the  rate  of  80955  miles  in  an  hour.  Venus 
forms  its  day,  or  turns  round  on  its  own  axis  in  23  h. 

22  m.  and  its  greatest  elongation  from  the  sun  is 
about  48^.  Like  Mercury,  it  is  invisible  at  midnight, 
and  is  only  seen  for  two  or  three  hours  in  the  morn- 
ing or  evening  when  it  passes  before  or  after  the  sun. 

The  Earth  is  placed  next  to  Venus  in  the  planetary- 
sphere;  its  diameter  is  7920  miles,  and  it  is  about  96 
millions  of  miles  distant  from  the  sun.  It  makes  one 
complete  revolution  in  its  orbit  in  365  d.  5  h.  48  m.; 
moving  at  the  rate  of  68856  miles  in  an  hour.  The 
earth  turns  round  its  own  axis  from  west  to  east  in 

23  h.  56  m.  which  produces  the  apparent  diurnal  mo- 
tion of  the  sun  and  all  the  heavenly  bodies  from  east 
to  west,  in.the  same  time ;  the  diurnal  motion  of  the 
earth  likewise  causes  what  we  call  the  rising  and  set- 
ting of  the  sun,  and  the  length  of  days  and  nights. 
The  axis  of  the  earth  is  inclined  23|°  to  the  plane  of 
its  orbit,  and  as  this  axis  is  always  parallel  to  itself, 
or  in  the  same  direction  in  every  part  of  its  course,  it 
causes  the  sun  at  one  time  of  the  year  to  enlighten 
more  of  the  northern  parts  of  the  globe,  and  at  ano- 


202  The  Sun  and  Planets. 

ther  time  of  the  southern,  which  produces  the  various 
seasons  of  spring,  summer,  autumn,  and  winter. 

The  Moon  is  a  secondary  planet,  and  an  attendant 
of  the  earth,  revolving  in  an  elliptical  orbit,  or  rather 
the  earth  and  the  moon  both  revolve  round  a  com- 
mon centre  of  gravity,  which  imaginary  point  is  as 
much  nearer  to  the  earth  as  the  mass  of  the  earth  ex- 
ceeds that  of  the  moon.  The  moon  makes  its  revolu- 
tion in  its  orbit  round  the  earth  in  27  d.  7  h.  moving 
at  the  rate  of  2299  miles  in  an  hour.  Its  time  in  go- 
ing round  the  earth,  reckoning  from  one  new  moon  to 
another,  or  when  it  overtakes  the  sun  again,  is  29  d. 
12  h.  It  is  2161  miles  in  diameter,  and  240000  miles 
distant  from  the  earth,  turning  round  its  own  axis  in 
the  same  time  that  it  revolves  round  the  earth,  so  that 
its  days  and  nights  are  of  the  same  length  as  our  lu- 
nar months.  The  moon's  orbit  is  inclined  to  the 
plane  of  the  ecliptic  in  an  angle  of  about  5^,  and 
crosses  it  in  two  opposite  points,  called  the  nodes: 
lunar  eclipses  take  place  when  the  moon  is  in  or  near 
these  points. 

Mars  is  the  first  of  the  superior  planets,  and  is 
placed  on  the  outside  of  the  earth's  orbit:  it  is  5309 
miles  in  diameter,  and  its  distance  from  the  suri  is 
about  146  millions  of  miles;  it  performs  its  revolution 
round  the  sun  in  1  y.  321  d.  23  h.  moving  at  the  rate 
of  55287  miles  in  an  hour,  and  revolves  round  its 
own  axis  in  24  h.  39  m.  This  planet  has  a  greater 
analogy  to  the  earth  than  any  other  planet:  the  diur- 
nal motion  and  the  obliquity  of  its  ecliptic  have  very 
small  variation  from  those  of  the  earth.  When  it  is  in 
opposition  to  the  sun  it  is  five  times  as  near  to  us  as 
when  it  is  in  conjunction,  which  has  a  very  visible 
effect  on  its  magnitude :  it  has  a  dusky  and  reddish 
hue,  which  is  supposed  to  arise  from  the  nature  of  the 
atmosphere  that  surrounds  it. 

Jupiter  is  a  primary  planet,  placed  between  Mars 
and  Saturn,  90228  miles  in  diameter,  and  it  is  about 


The  Siin  and  Planets,  203 

a  thousand  times  bigger  than  the  earth :  its  distance 
from  the  sun  is  499750000  miles,  and  it  revolves 
round  its  own  orbit  in  11  y.  314  d.  12  h.  moving  at 
the  rate  of  29000  miles  an  hour:  it  has  a  diurnal  mo- 
tion round  its  axis  in  9  h.  S6  m.  which  carries  the 
equatorial  parts  of  its  surface  with  a  velocity  of  25000 
miles  an  hour;  this  is  about  twenty-five  times  faster 
than  the  revolution  of  the  same  parts  of  the  earth. 
Jupiter  has  four  satellites  revolving  round  it,  which 
enlighten  it  in  the  absence  of  the  sun,  as  the  moon 
enlightens  the  earth;  beside  these  attendants,  it  i^ 
surrounded  by  faint  bodies,  which  are  called  its  zones 
or  belts ;  these  appearances  are  frequently  changing, 
and  are  ascribed  to  clouds  in  its  atmosphere.  As  the 
axis  of  Jupiter  is  nearly  perpendicular  to  its  orbit, 
there  is  hardly  any  difference  in  the  seasons,  and  the 
days  and  nights  are  five  hours  each. 

Saturn  is  the  sixth  primary  planet,  and  has  been 
considered  for  many  ages  as  the  last  and  most  remote 
planet  in  our  system  until  some  recent  discoveries. 
In  consequence  of  Saturn's  immense  distance  from 
the  earth,  it  casts  but  a  feeble  light  of  a  dusky  colour, 
although  it  surpasses  all  the  rest,  Jupiter  excepted, 
in  actual  magnitude.  Its  diameter  is  computed  to  be 
79979  miles,  and  its  distance  from  the  sun 
916500000  miles,  which  is  near  ten  times  the  dis- 
tance that  the  earth  is  from  the  same  luminary.  It 
takes  29  y.  167  d.  to  make  one  complete  revolution 
in  its  orbit,  and  moves  at  the  rate  of  22298  miles  in 
an  hour.  This  planet  is  surrounded  by  two  rings, 
one  within  ihe  other,  and  beyond  these  rings  are  se- 
ven attendant  moons,  two  of  which  were  discovered 
by  Herschel. 

Georgium  Sidiis,  This  planet  was  discovered  by 
Herschel  in  the  year  1781:  its  light  is  of  a  bluish 
white  colour,  and  may  sometimes  be  seen  by  the 
naked  eye  in  a  clear  night,  without  moonlight.  The 
time  of  its  annual  revolution  is  about  80  years,  and 


204.  The  Sun  and  Planets, 

its  diameter  34299  miles,  which  is  more  than  four 
times  the  diameter  of  the  earth :  its  distance  from  the 
sun  is  about  1832  millions  of  miles,  and  it  has  an 
inclination  of  43^  35'  to  its  orbit. 

To  assist  the  memory  and  form  an  idea  of  the  pro- 
portional distance  of  each  planet  from  the  sun ;  if  the 
greatest  extent  of  the  Georgium  Sidus  from  the  sun 
were  divided  into  190  parts,  the  proportional  dis- 
tance of  the  rest  of  the  orbits  would  be,  Mercury  5, 
Venus  7,  the  Earth  10,  Mars  15,  Jupiter  52,  and 
Saturn  95. 


205 


The  Earth  and  Moon  distinctly  considered;  with  an 
explanation  of  Seasons  and  Eclipses. 

The  Earth. 

In  the  early  ages,  the  opinions  of  mankind  were 
much  divided  concerning  the  form  of  the  earth;  some, 
being  guided  by  visual  appearance,  conceived  it  to  be 
a  stationary  plane,  bounded  by  the  horizon,  and  that 
the  whole  universe  was  contained  in  that  part  of  the 
heavens  which  was  presented  to  their  view.  How- 
ever ill  conceived  this  opinion  may  seem  in  the  pre- 
sent day,  it  has  had  its  supporters,  even  in  the  ages 
of  Christianity.  But  as  a  strong  proof  that  all  the 
sages  of  antiquity  were  not  equally  ignorant  of  its 
real  form,  we  find  the  ancient  Babylonians  and  Greeks 
calculated  eclipses  both  of  the  sun  and  the  moon, 
which  may  be  taken  as  a  fair  argument  to  show  that 
they  were  not  unacquainted  with  the  rotundity  of  the 
earth. 

The  number  of  convincing  proofs  which  are  pro- 
duced in  the  present  day,  cannot  leave  a  doubt  of  its 
globular  form,  even  in  the  commonest  minds:  for 
those  who  stand  on  the  seashore,  and  observe  a  ship 
making  out  for  sea,  will  perceive  the  hull  first  de- 
cline, as  it  approaches  the  horizon,  till  it  totally  dis- 
appear, and  leave  the  mast  and  sails  still  in  sight, 
and  these  gradually  decline  till  the  top  of  the  mast 
sink  from  the  eye ;  even  then,  if  the  spectator  as- 
cend the  top  of  a  hill  or  building,  he  will  perceive 
the  vessel  again,  till  the  convexity  again  hide  it  from 
his  sight. 

It  is  perfectly  clear,  that  if  the  earth  were  a  plane, 
the  hull  of  the  vessel,  which  is  the  largest  part  of  the 
body,  would  be  seen  the  longest,  and  the  mast  and 
sails  would  first  disappear  as  the  inferior  objects  of 

2B 


'206  The  Earth, 

sight:  but  observation  proves  the  reverse:  then  what 
else,  than  the  sphericity  of  the  earth,  can  produce 
this  effect? 

When  a  ship  is  sailing  at  sea,  either  northwards  or 
southwards,  those  stars  which  are  placed  nearly  op- 
posite to  the  poles  of  the  earth,  appear  to  have  no 
diurnal  motion  but  remain  fixed  in  the  extreme 
parts  of  the  heavens:  therefore,  if  the  earth  were  a 
plane,  considering  the  immense  distance  of  the  stars, 
a  ship  in  sailing  either  directly  north  or  south  would 
still  observe  them  under  the  same  angle,  or  with  the 
same  altitude;  but  daily  experience  teaches  us  the 
contrary,  for  vessels  sailing  northwards  observe  a 
gradual  elevation  of  those  stars  which  are  in  the  north 
polar  regions,  and  a  depression  of  those  towards  the 
south :  in  sailing  southwards,  the  appearance  is  the 
reverse,  for  the  southern  stars  are  elevated  and  the 
others  depressed.  This  appearance  is  rationally  ex- 
plained by  the  convexity  of  the  earth,  which  increases 
the  angle  of  observation  as  the  ship  sails  towards  the 
star,  and  decreases  it  as  the  vessel  moves  the  oppo- 
site  way. 

Eclipses  of  the  moon  are  caused  by  the  earth's  sha- 
dow falling  upon  it,  when  the  earth's  body  is  inter- 
posed between  the  sun  and  the  moon;  yet,  we  always 
iind,  that,  in  whatever  position  the  earth  may  be 
placed  at  that  time,  its  shadow  falls  with  a  circular 
edge  upon  the  disk  of  the  moon,  which  could  not 
always  happen  if  the  earth  were  not  of  a  globular  form. 

The  irregularities  on  the  surface  of  the  earth  have 
no  visible  effect  upon  its  shadow  on  the  moon,  for  the 
highest  mountain  on  its  surface,  considering  the  mag- 
nitude of  the  earth  and  its  distance  from  the  moon, 
would  cause  no  more  visible  effect  in  its  shadow,  thim 
the  finest  grain  of  sand  would  produce  on  that  of  a 
billiard  ball. 

The  globular  form  of  the  earth  is  likewise  practi- 
cally known  by  those  circumnavigators  who  have 


Motion  of  the  Earth.  207 

sailed  round  it,  by  always  continuing  an  easterly  or 
westerly  course,  which  has  brought  them  again  to 
the  same  port  whence  they  set  out.    - 

These  and  various  other  proofs  in  the  higher  de- 
partments of  science,  leave  no  doubt  of  the  sphericity 
of  the  earth,  although  it  differs,  in  some  measure, 
from  a  sphere,  as  it  is  flatted  towards  its  poles,  some- 
thing resembling  an  orange,  so  that  the  diameter  of 
the  earth  at  the  equator  exceeds  that  at  the  poles  by 
about  thirty-four  miles :  this  was  discovered,  by  ob- 
serving that  a  pendulum  moved  slower  as  it  approach- 
ed the  equator,  and  faster  as  it  advanced  towards  the 
poles ;  and  this  difference  is  caused  by  the  centrifugal 
motion  of  the  earth  on  its  axis,  which  diminishes  the 
force  of  gravity  towards  the  equatorial  parts  of  the 
globe,  and  flattens  the  earth  towards  its  polar  extre-- 
mities. 

Motion  of  the  Earth, 

Independently  of  a  small  motion  which  occa- 
sions what  is  called  the  precession  of  the  equinoxes, 
the  earth  has  two  general  motions ;  one  round  its  own 
axis  in  twenty-four  hours,  which  is  called  its  diurnal 
motion,  and  causes  the  succession  of  day  and  night; 
the  other  is  its  annual  motion  or  revolution  round 
the  sun  as  its  centre,  keeping  its  axis  always  inclined 
to  its  path  in  an  angle  of  about  23^  degrees,  which 
produces  the  various  seasons  of  spring,  summer,  au- 
tumn, and  winter. 

To  the  visual  sense  it  would  seem  that  the  earth  is 
fixed  as  the  centre  of  our  planetary  system,  and  that 
the  sun  and  the  rest  of  the  celestial  bodies  have  a  daily 
motion  round  it.  It  is  not  extraordinary  that  men  in 
early  ages  should  have  considered  this  as  the  system 
of  planetary  motion,  when,  at  the  present  hour,  the 
uninformed,  who  judge  only  from  sight,  will  not  be 
persuaded  to  give  up  their  opinions;  but  to  those  who 


?08  Motion  of  the  Earth. 

are  susceptible  of  conviction,  there  are  so  many  proofs 
of  this  error,  that  nothing  but  ignorance  or  obstinacy 
can  hesitate  to  beUeve  them. 

The  following  observation  may  tend  to  show  re- 
lative motion,  and  how  easily  our  senses  may  be  de- 
ceived. 

If  a  person  be  placed  under  the  deck  of  a  vessel 
when  it  is  sailing  gently  down  the  side  of  a  coast  with 
a  fair  wind,  he  will  be  perfectly  insensible  of  its  mo- 
tion; but  if  he  cabt  his  eyes  on  the  shore,  he  will  see 
all  the  objects  pass  him  with  a  rapidity  equal  to  the 
velocity  of  the  vessel,  and  the  vessel  itself  will  be  ap- 
parently at  rest.  But  here  our  reason  tells  us  rhat  our 
senses  are  deceived,  and  that  the  motion  is  in  the  ship 
and  not  in  the  objects.  Then  why  cannot  we  suffer 
ourselves  to  believe  that  our  sight  is  deceived  by  the 
apparent  motion  of  the  sun,  and  simplify  the  system 
of  the  universe  by  admitting  a  diurnal  revolution  of 
the  earth  from  west  to  east,  rather  than  force  such  a 
monstrous  hypothesis  as  would  drive  the  whole  uni- 
verse round  the  earth  from  east  to  w^est  to  form  the 
period  of  a  day. 

By  the  diurnal  motion  of  the  earth,  the  same  phe- 
uomena  appear  as  if  all  the  celestial  bodies  turned 
round  it:  so  that  in  its  rotation  from  west  to  east, 
when  the  sun  or  a  star  just  appears  on  the  eastern 
side  of  the  horizon,  it  is  said  to  be  rising,  and,  as  the 
earth  continues  its  revolution,  it  seems  gradually  to 
ascend  till  it  has  reached  the  meridian,  which  is  di- 
rectly south  of  the  observer;  here  the  object  has  its 
greatest  elevation,  and  begins  to  decline  till  it  set, 
or  become  invisible  on  the  western  side.  In  the 
same  manner  the  sun  appears  to  rise  and  run  his 
course  to  the  western  horizon,  where  he  disappear^ 
and  night  ensues,  till  he  again  illuminate  the  same 
part  of  the  earth  in  another  diurnal  revolution. 

All  the  heavenly  bodies  do  not  appear  to  rise  and 
set ;  those  that  are  placed  in  the  two  poles,  or  are  op- 


Motion  of  the  Earth.  209 

posite  to  the  imaginary  axis  of  the  earth,  can  have  no 
apparent  motion  from  its  daily  revolution,  therefore  al- 
ways appear  in  the  same  part  of  the  heavens. 

In  summing  up  the  diurnal  motion  of  the  earth,  it  is 
considered  as  a  globular  opake  body  turning  round  on 
an  imaginary  axis  from  west  to  east,  and  that  it  is  en- 
lightened by  the  sun's  rays,  which  perpetually  illumi- 
nate one  half  of  its  surface :  the  imaginary  great  circle 
which  separates  the  illuminated  part  from  that  which  is 
dark,  or  turned  from  the  sun's  rays,  is  called  the  termi- 
nator, and  when  any  point  on  the  eastern  side  of  the 
globe  comes  to  this  line,  it  is  sunrise,  and  when  the 
point  advances  to  the  edge  of  the  terminator  on  the  op- 
posite or  western  side,  it  is  called  sunset.  The  light 
which  gradually  appears  before  the  rising  of  the  sun  and 
gradually  decreases  after  sunset,  is  called  the  crepus- 
culum,  or  twilight,  and  is  occasioned  by  the  reflection 
of  the  sun's  rays  from  the  atmosphere  that  surrounds 
the  earth. 

Before  we  speak  of  the  annual  motion  of  the  sun,  it 
may  be  seen,  by  the  following  diagram,  that  if  the 
earth  be  at  rest  and  the  sun  in  motion,  or,  if  the  sun  be 
at  rest  and  the  earth  in  motion,  the  same  general  ap- 
pearance and  effect  will  be  produced;  that  is,  either  of 
them  would  apparently  describe  a  great  circle  in  the 
heavens  in  the  plane  of  the  ecliptic. 

For,  if  the  earth  be  supposed  at  rest,  and  the  sun  re- 
volving round  it,  in  the  orbit  a  b  c  d,  it  would  appear 
to  a  spectator  on  the  earth,  to 
describe  the  great  circle  f  e  g  h 
in  the  heav^ens ;  for,  if  a  spectator 
on  the  earth  at  o  view  the  sun  at 
A,  it  will  be  referred  to  e,  and  at 
B  to  F,  and  so  on  of  the  rest. 

But,  if  the  sun  be  placed  at 
o,  and  the  earth  in  the  orbit 
A  B  G  p;  when  the  earth  is  at  a, 
the  sun  will  appear  in  a  great 


210  Changes  of  the  Seasons. 

circle  of  the  heavens  at  h  ;  when  it  is  at  b  the  sun  will 
be  seen  at  g,  and  so  on  with  the  other  two  points. 

Therefore,  whatever  regards  the  sun's  place,  with 
respect  to  its  appearance  in  the  heavens,  it  may  be  con- 
sidered, instead  of  the  earth,  as  moving  in  an  infinitely- 
great  circle,  called  the  ecliptic,  having  its  centre  in  the 
eye  of  the  observer. 

Changes  of  the  Seasotis, 

The  earth  is  s'lpposed  to  be  divided  into  two  equal 
parts,  by  a  great  circle  drawn  at  equal  distances  from 
the'  poles,  which  is  called  the  equator :  smaller  circles 
drawn  parallel  to  the  equator,  approaching  the  poles, 
are  called  the  parallels  of  latitude,  and  great  circles  in- 
tersecting the  equator  at  right  angles  and  passing 
through  the  poles,  are  called  meridians  of  longitude. 
The  ecliptic  is  the  earth's  orbit^  or  the  apparent  annual 
course  of  the  sun,  making  an  angle  of  about  23i  degrees 
with  the  equator,  from  which  the  various  seasons  are 
derived,  and  the  terminator  is  a  great  circle  which 
bounds  the  illuminated  part  of  the  earth,  or  that  half  qf 
its  surface  which  is  always  turned  towards  the  sun. 

Let  A  B  c  D  represent  the  earth,  with  its  poles  a  b, 
so  placed,  that  the  terminator 
of  the  sun's  rays  passes 
through  each  pole;  then  it 
will  likewise  divide  every 
circle  or  parallel  of  latitude  Ji 
^,  ^,  f,  &c.  into  two  equal  C] 
parts,  so  that  one  half  will 
be  enlightened  by  the  sun's 
rays,  and  the  other  will  be 
dark  or  turned  from  them; 
but  during  the  diurnal  mo- 
tion, every  part  of  each  circle,  or  each  meridian,  will  be 
brought  to  the  terminator,  and  carried  through  the  illu- 
minated part  in  the  same  time,  which  makes  the  days 


Changes  of  the  Seasons,  211 

and  nights  of  equal  length  on  every  part  of  the  globe, 
except  at  the  poles  a  and  b,  where  the  sun  would  be 
seen  during  the  whole  day,  just  skimming  above  the 
horizon. 

But  if  the  axis  of  the  earth  a  b  do  not  coincide  with 
the  great  circle  of  the  termi-  ^y^ 

nator  e  f;   the  great  circle  "^ 

c  D,  called  the  equator,  will 
be  divided  into  t^o  equal 
parts  at  g,  and  the  inhabi- 
tants under  that  line,  will 
have  their  days  and  nights 
of  equal  length;  but  the  pa- 
rallels aa,  b  b,  c  c,  he,  will 
be  divided  into  unequal 
parts  by  the  terminator  e  f, 
and  the  inhabitants  under  those  parallels  which  have  the 
greatest  part  of  their  circle  illuminated,  will  have  their 
days  longer,  and  nights  shorter,  than  those  which  lie 
on  the  opposite  side  of  the  equator,  where  the  dark  and 
illuminated  parts  are  just  the  reverse.  It  is  likewise 
clear,  from  inspection,  that  as  the  diurnal  and  nocturnal 
parts  of  these  parallels  respectively  increase  from  the 
equator,  those  places  that  lie  under  them  will  have  a 
greater  disproportion  of  day  and  night,  so  that,  those 
that  are  at  or  near  the  pole  a,  will  have  constant  day,  or 
the  sun  always  above  the  horizon ;  and  those  at  the  op- 
posite pole  B,  will  have  continual  darkness,  or  the  sun 
always  beneath  the  horizon,  for  six  months  together. 

In  the  present  position  of  the  axis,  those  who  lie  on 
the  northern  side  of  the  equator  have  their  summer,  and 
those  on  the  southern,  their  winter. 

The  difference  of  light  is  not  the  only  cause  of  sum- 
mer and  winter.  The  sun  appears  much  higher  above 
the  horizon  to  those  places  which  have  the  longest  days, 
consequently  the  rays  fall  more  perpendicularly  upon 
the  earth,  which,  joined  to  the  superior  quantity  of  heat 
that  is  communicated  by  the  greater  length  of  the  day, 


212  Changes  of  the  Seasons. 

day,  causes  the  summer  to  be  much  liotter  than 
winter. 

Having  shown  that  the  length  of  day  and  night  is 
produced  by  the  different  relations  of  the  axis  of  the 
earth  to  the  terminator,  we  will  next  explain  how  this 
varies  at  different  times  of  the  year,  and  produces  the 
variety  of  seasons. 

We  find,  by  common  observation,  that  the  sun  de- 
scribes different  parallels,  or  daily  appears  at  different 
heights  above  the  horizon,  which  shows,  that  the  plane 
of  the  ecliptic,  or  sun's  apparent  path,  does  not  coincide 
with  the  plane  of  the  equator,  otherwise  the  sun  would 
have  the  same  altitude  daily,-'  and  days  and  nights  would 
always  be  of  an  equal  length.  Let  it  be  remembered, 
that  the  effect  is  the  same,  whether  the  motion  be  in  the 
earth  or  the  sun ;  therefore,  we  more  properly  conclude, 
that  the  axis  of  the  earth  is  inclined  to  its  orbit,  or  that 
the  plane  of  the  equator  does  not  coincide  with  the  plane 
of  the  ecliptic.  Now  it  is  found  by  observation,  that  this 
inclination  of  the  planes  forms  an  angle  of  23^  degrees, 
which  remains  invariable  throughout  the  whole  of  the 
earth's  annual  course,  or  that  the  axis  of  the  earth  al- 
ways moves  parallel  to  itself. 

Therefore,  if  the  earth's  axis  be  inclined  to  the  plane 
of  the  ecliptic,  as  in  the  last  figure,  one  of  its  poles  will 
be  towards  the  sun,  and  the  other  will  be  removed  from 
it  in  an  equal  proportion:  but  when  the  earth  has  made 
half  of  its  revolution,  and  has  arrived  in  the  opposite 
point  of  its  orbit,  still  retaining  the  inclination  of  its 
axis,  the  pole  which  was  towards  the  sun  will  now  be 
removed  from  it,  and  the  opposite  pole  which  was  dark- 
ened, will  be  presented  to  it.  As  the  earth  passes  from 
one  of  tliese  extremes  to  the  other,  the  plane  of  the 
ecliptic  wdll  coincide  with  the  plane  of  the  equator,  near 
the  intermediate  distance  between  the  extreme  points', 
and  then  the  terminator  passes  through  the  two  poles  of 
the  earth,  as  in  figure  the  first. 

From  this  revolution  the  seasons  are  produced;  for 


Changes  of  the  Seasons, 


21: 


when  the  northern  pole  is  the  most  inclined  towards  the- 
sun,  it  is  midsummer  to  all  the  inhabitants  on  the  north 
side  of  the  equator;  who  then  have  their  longest  day, 
and  shortest  night:  but  when  the  earth  is  in  the  oppo- 
site pare  of  the  ecliptic,  or  180*^  distant,  then  the  soudi- 
ern  pole  has  its  greatest  inclination  to  the  sun,  and  the 
inhabitants  on  that  side  of  the  equator  have  their  mid- 
summer and  longest  dciys,  while  those  in  the  norUiern 
hemisphere  have  their  shortest  days  and  midwinter. 
When  the  earth  is  in  the  intermediate  part  of  its  orbit, 
or  90 "^  distant  from  either  extreme,  that  is,  when  the 
plane  of  the  ecliptic  coincides  with  the  equator,  the  days 
and  nights  are  of  equal  length  all  over  the  world,  which 
is  in  the  vernal  and  autumnal  equinoxes,  or  spring  and 
autumn. 

The  following  experiment  will -show  the  effects  which 
are  produced  by  the  inclination  of  the  earth's  axis  to  its 
orbit  through  all  the  twelve  signs  of  the  zodiac. 

Let  the  frame  c  p  represent  the  elliptical  orbit  of  the 
earth,  which  inter- 
sects   the   equator 
A   B    in    the    two 
points,    or    nodes 
E  F ;  making  an  an- 
gle of  23i^,  which  Hj 
is  the  inclination  of 
the  earth's  axis  to 
the  ecliptic;  and  let 
s  be  a  lighted  can-  ^ 
die,   placed  in  the 
centre  of  the  frame, 
representing      the 
sun  in  the  middle 
of     the    planetary 
system;    with   the 
ecliptic  divided  in- 
to signs,  and  the  corresponding  month  marked  against 
each. 

2C 


214  Changes  of  the  Seasons. 

If  a  small  globe,  or  terrella,  representing  the  earthy 
be  suspended  by  a  string  from  e,  or  Libra,  where  the 
circles  intersect  each  other,  and  the  eye  be  placed  a 
little  above  the  light  in  the  centre,  the  hemisphere  of 
the  globe  will  be  illuminated,  including  both  poies ;  the 
apparent  situation  of  the  sun  will  be  in  the  opposite 
side  of  the  ecliptic  f,  or  in  the  sign  Aries,  and  this  is 
called  the  vernal  equinox,  m  hich  kippens  about  the  20th 
of  March,  when  the  terminator  passes  through  the  poles, 
and  makes  the  days  and  nights  equal  all  over  the  earth. 
Whilst  the  terrella  is  moving  through  Libra,  Scorpio, 
and  Sagittarius,  the  terminator,  or  edge  of  the  light, 
keeps  increasing  beyond  the  upper  or  north  pole ; 
where  it  has  attained  its  greatest  distance,  then  the  globe 
is  in  Capricornus,  g,  and  the  sun  is  apparently  in  the 
opposite  sign  Cancer,  "h,  which  happens  on  the  21st  of 
June.  Now  all  the  parallels  of  latitude  in  the  northern 
hemisphere  have  the  greatest  part  of  their  circles  illu- 
minated, and  their  days  are  the  longest ;  but  the  dura- 
tion of  light,  or  the  length  of  day,  is  in  proportion  to 
the  distance  of  the  parallels  from  the  equator,  increas- 
ing from  it  to  the  nordi  pole,  and  decreasing  in  like  ra- 
tio from  the  equator  to  the  south  pole. 

But  with  respect  to  the  terrella.  Whilst  we  trace  the 
return  of  the  terminator  towards  the  north  pole  through 
Capricornus,  Aquarius,  and  Pisces,  we  perceive  it 
advance  towards  the  south,  till  the  terrella  is  in  the  be- 
ginning of  Aries,  and  the  apparent  place  of  the  sun  is 
in  Libra,  then  the  terminator  passes  through  the  poies, 
and  the  days  and  nights  are  again  of  an  equal  length, 
which  happens  at  the  autumnal  equinox,  about  the  22d 
of  September.  Let  the  terrella  be  moved  on  through 
Aries,  Taurus,  and  Gemini,  till  it  reach  the  first 
degree  in  Cancer,  and  the  sun's  apparent  place  will  be 
in  Capricornus;  during  its  progress  through  these 
signs,  the  illuminated  part,  or  the  terminator,  leaves 
the  northern  pole  in  darkness,  and  enlightens  the  re- 
gions about  the  southern.  Now  the  greater  part  of  the 


Changes  of  the  Seasons. 


215 


circles  or  parallels  of  latitude  in  the  northern  hemi- 
sphere lYt  in  darkness,  whilst  those  in  the  southern 
have  their  greatest  portion  of  light,  which  produces 
midwinter  to  the  former,  and  midsummer  to  the  latter; 
and  this  takes  place  about  the  21st  of  December.  By 
moving  the  globe  through  the  three  remaining  signs, 
Cancer,  Leo,  and  Virgo,  the  terminator  again  ap- 
proaches towards  the  north,  illuminates  both  poles, 
regains  its  first  position  in  Libra,  and  completes  its 
annual  revolution. 

What  is  usually  called  summer,  that  is,  from  the 
vernal  till  the  autumnal  equinox,  is  nearly  eight  days 
longer  than  from  the  autumnal  till  the  vernal;  for  the 
sun  in  passing  through  the  six  northern  signs  Aries, 
Taurus,  Gemini,  Cancer,  Leo,  and  Virgo,  performs 
its  apparent  motion  in  186d.  llh.  51m,;  but  in  passing 
through  the  winter  signs  of  Libra,  Scorpio,  Sagitta- 
rius, Capricornus,  Aquarius,  and  Pisces,  it  only  takes 
up  178d.  17h.  58m.  which  makes  a  difference  of  7d. 
17h.  53m. 

Let  A  B  c  D  represent  the  earth's  orbit,  and  s  the 
sun  in  one  of  its  foci;  when 
the  earth  is  at  c  the  sun  ap- 
pears at  H  the  first  sign  Aries, 
and  as  the  earth  moves 
through  c  B  to  d,  the  sixy, 
southern  signs;  the  sun  ap- 
pears to  move  through  the 
six  northern  h  e  r.  In  like 
manner,  whilst  the  earth  pas- 
ses through  the  northern 
signs  D  A  c,  the  sun  passes  through  the  southeni"jF  g  h, 
the  corresponding  circle  in  the  heavens  to  half  of  the 
earth's  orbit  c  b  d.  Thus  the  line  f  h  divides  the 
ecliptic  into  two  equal  parts,  and  the  elliptical  orbit  of 
the  earth  into  two  unequal  parts ;  the  gi'eater  part  c  a  r>^ 
is  that  which  the  earth  describes  in  summer,  and  the 


216  The  Moon's  Motion, 

less  is  its  Avinter  course.  Beside  these  unequal  divisions 
of  the  earth's  orbit,  it  apparently  moves  slower  in  its 
aphelion,  or  the  distant  part  of  its  course,  than  in  its 
perihelion,  or  when  it  is  nearest  the  sun. 

1  he  apparent  diameter  of  the  sun  is  greater  in  win- 
ter than  in  summer,  for  the  sun  is  considerably  nearer 
the  earth  whilst  the  earth  passes  through  the  winter 
signs  c  B  D,  than  whilst  it  passes  through  the  summer 
signs  D  A  c. 

It  may  naturally  be  asked,  why  the  winter  is  colder 
than  the  summer,  as  the  sun  is  nearer  the  earth?  In 
summer,  as  before  observed,  the  sun  rises  much  higher 
above  the  horizon,  therefore,  its  rays  fall  in  a  greater 
quantity,  and  more  directly  upon  the  earth  than  in 
winter;  likewise  the  length  of  day,  or  the  time  that  the 
sun  is  above  the  horizon  in  summer,  being  much  longer 
than  in  winter,  the  earth  and  atmosphere  receive  more 
heat  in  the  day  than  they  lose  in  the  night,  so  that  we 
have  a  gradualaccumulationof  heatduriiig  the  summer 
months,  which  makes  it  generally  hotter  after  the  sun 
has  passed  the  summer  solstice,  or  tropic  of  Cancer, 
than  in  any  of  the  preceding  signs. 

The  Moon's  Motion, 

The  moon  is  one  of  those  heavenly  bodies,  which 
we  call  satellites ;  it  is  secondary  to  the  earth,  and  re- 
volves round  it,  whilst  the  earth  performs  its  annual 
course. 

The  moon's  apparent  place,  viewed  by  a  spectator 
on  the  earth,  is  extended  to  a  great  circle  of  the  heavens, 
and  seems  to  move  through  the  twelve  signs  of  the  zo- 
diac, in  a  month  or  lunar  day. 

The  plane  of  the  moon's  orbit,  if  it  were  extended, 
would  intersect  the  ecliptic  in  two  points,  making  an 
angle  with  it  of  about  five  degrees ;  but  this  inclination 
vaines,  being  greatest  when  the  moon  is  in  its  quadra- 


The  Moon's  Motion,  217 

tiire,  and  least  when  it  is  in  conjunction  or  opposition 
with  the  sun. 

The  two  points  where  the  moon's  orbit  cuts  the 
ecliptic  are  called  the  nodes:  when  the  moon  ascends 
from  the  south  to  the  north  side  of  the  ecliptic,  it  is 
called  the  ascending  node,  and  from  the  north  to  the 
south,  the  descending  node.  The  line  of  the  nodes  is 
not  always  directed  to  the  same  point,  but  has  a  motion 
contrary  to  the  order  of  the  signs,  and,  by  this  retro- 
grade course,  it  completes  its  circuit  in  18y.  225d.,  at 
which  time  the  line  of  the  nodes  returns  to  the  same 
point  in  the  ecliptic. 

When  the  moon  crosses  the  ecliptic,  it  is  in  its 
nodes,  but  in  all  other  parts  of  its  orbit  it  is  in  north  or 
south  latitude,  according  as  it  is  above  or  below  the 
ecliptic. 

The  mean  time  of  a  revolution  of  the  moon  about 
the  earth,  that  is,  from  one  new  moon  to  another,  is 
called  a  sy nodical  month,  or  lunation,  and  consists  of 
29d.  12h.  44m.  The  line  of  its  revolution  round  the 
earth,  from  any  point  in  the  zodiac  to  the  same  point 
again,  is  called  a  periodical  month,  and  contains  27d. 
7h.  43m.  The  moon  moves  in  its  orbit  about  2290 
miles  in  an  hour,  and  only  turns  once  round  its  own 
axis  whilst  it  makes  a  revolution  round  the  earth, 
which  causes  it  always  to  present  the  same  side  towards 
the  earth,  and  makes  its  day  and  night  of  the  same 
length  as  our  lunar  month. 

If  the  earth  Avere  stationary,  the  periodical  and  sy« 
nodical  months  would  be  the  same;  but  as  the  earth 
keeps  moving  forwards  in  its  orbit,  whilst  the  moon  is 
performing  its  revolution,  it  has  not  only  to  pass 
through  its  own  orbit,  but  has  likewise  to  overtake  the 
earth  again  in  its  passage  through  the  ecliptic. 


218  Phases  of  the  Moon . 

For  if  s  be  the  sun  in  the  centre  of  the  system;  e 
part  of  the  earth's  orbit,  and 
M  A  the  orbit  of  the  moon  ; 
when  the  moon  is  in  conjunc- 
tion  at  A,  if  the  earth  remain-  ^ 
ed  at  B,  whilst  it  made  its 
revolution  a,  a,  m,  by  a,  the 
periodical  and  synodical 
months  would  be  the  same: 
but,  during  this  revolution, 

the  earth  has  passed  on  in  its  orbit  to  c  ;  therefore  the 
moon  must  advance  to  c,  before  the  earth  and  moon 
can  come  into  conjunction  again;  but  it  is  obvious,  by 
inspection,  that  when  the  moon  has  arrived  at  f,  it  will 
have  completed  its  revolution  round  its  orbit,  and  the 
time  of  performing  the  remaining  arc  f  c,  will  be  the 
difference  of  time  between  the  periodical  and  synodical 
month,  which  is  about  two  days  and  five  hours. 

Phases  of  the  Moon,       ^ 

The  moon  is  a  dark  opake  body  moving  round  the 
earth  in  a  small  orbit,  and  shines  by  a  borrowed  light 
from  the  sun,  which  illuminates  one  half  of  its  body, 
and  leaves  the  other  in  darkness.  We  perceive  differ- 
ent degrees  of  this  illumination,  according  to  the  vari- 
ous positions  of  the  moon,  with  respect  to  the  sun  and 
the  earth:  hence  we  see  one  half  of  its  body  enlighten- 
ed, or  a  full  face,  when  it  is  placed  in  opposition,  or 
in  that  part  of  its  orbit  which  is  the  most  remote  from 
the  sun.  When  the  moon  is  in  conjunction,  or  in  that 
part  of  its  orbit  which  is  between  the  earth  and  the  sun, 
its  enlightened  face  is  turned  from  us,  which  renders  it 
invisible;  this  is  the  time  of  new  moon.  When  the 
moon  appears  in  the  intermediate  part  of  its  orbit,  be- 
tween the  conjunction  and  opposition,  it  is  in  its  quad- 
ratures, and  about  half  of  its  illuminated  surface  is 


Phases  of  the  Moon.  2 1^ 

turned  towards  us.  Its  phases  and  appearances  are  par- 
ticularly explained  by  the  figure. 

Let  s  represent  the  sun,  k  the  earth,  a  b  c  d,  &c. 


the  moon  in  its  orbit,  with  the  sun's  rays  falling  on 
that  half  of  its  surface  which  is  opposite  to  the  sun, 
and  the  outer  circle  c,  b,  c,  d,  &c.  the  various  phases 
of  the  moon,  as  they  appear  to  a  spectator  on  the  earth, 
during  the  whole  of  a  lunar  month. 

When  the  moon  is  in  conjunction  with  the  sun  at 
A,  the  darkened  side  is  presented  towards  the  earth; 
therefore,  being  an  opake  body,  it  becomes  invisible, 
and  it  is  then  called  the  time  of  new  moon:  when  it 
has  passed  an  eighth  of  its  orbit  and  arrived  at  b,  a 
quarter  of  its  enlightened  surface  will  be  turned  to- 
wards the  earth,  and  it  will  appear  horned  as  at  b.  In 
passing  another  eighth  of  its  orbit,  it  arrives  at  c,  and 


220  Eclipses  of  the  Moon. 

it  is  then  in  it^  quadrature ;  one  half  of  its  surface  ap- 
peared illuminated  at  c;  after  having  passed  on  to  d, 
it  appears  gibbous,  and  more  than  one  half  of  its  face 
is  illuminated  as  appears  at  d;  at  e,  the  whole  face  is 
seen  bright,  which  is  called  opposition,  or  the  full 
moon.  Thus,  after  having  attained  its  fullest  appear- 
ance, it  again  begins  to  decline  from  e  to  f,  and  ap- 
pears gibbous  atyV  thence  it  passes  to  g  its  quadrature, 
and  is  seen  at  g,  half  illuminated,  then,  after  being 
horned  at  /z,  it  completes  its  revolution  and  falls  into 
conjunction  at  a,  with  the  sun. 

The  earth  serves  to  enlighten  the  moon,  in  the 
same  manner  as  the  moon  enlightens  us;  but  its  ap- 
pearance must  be  much  larger  than  that  of  the  moon 
to  the  earth,  and  the  changes  take  place  in  contrary 
order ;  that  is,  when  the  moon  appears  full  to  us,  the 
earth  must  be  in  conjunction  with  the  sun,  which 
turns  the  darkened  surface  to  the  lunarians. 

Soon  after  the  new  moon,  the  whole  body  is  dimly 
seen,  independently  of  the  illuminated  crescent  on  its 
outer  surface,  which  proceeds  from  the  light  that  is 
reflected  on  it  from  the  earth;  for  at  our  new  moon  the 
earth  appears  as  a  full  moon  to  the  lunarians,  and  part 
of  the  light  whidi  they  receive  from  us,  is  again  re- 
flected back  to  the  earth. 

Eclipses  of  the  Moo?i, 

An  eclipse  of  the  moon  is  a  privation  of  light, 
caused  by  the  interposition  of  the  earth  directly  be- 
tween the  sun  and  the  moon,  vvhich  intercepts  the 
sun's  rays,  and  prevents  them  from  illuminating  her 
surface.  Or  it  may  be  considered  as  proceeding  from 
the  conical  shadow  of  the  earth,  when  the  moon  enters 
between  the  base  and  the  vertex.  As  the  earth's  orbit 
is  in  the  plane  of  the  ecliptic  when  it  is  viewed  from 
the  sun,  it  is  evident  that  the  earth's  shadow  must  tend 


Eclipses  of  the  Moon.  221 

directly  to  that  part  of  the  heavens ;  and  as  the  moon's 
orbit  has  an  inchnation  of  about  five  degrees  with  the 
ecHptic,  and  only  crosses  it  in  two  points,  called  its 
nodes;  the  shadow  of  the  earth  cannot  fall  upon  th^p 
moon,  except  it  is  in  or  near  one  of  its  nodes. 

Let  the  line  a  d  represent  a  part  of  the  ecliptic,  the 
plane  of  which  coincides  with  that  of  the  earth's  orbit, 
and  c  B  part  of  the  orbit  of  the  moon,  crossing  the 


ecliptic  at  h,  which  is  called  its  node.  Then  if  e  f  g  h 
represent  the  earth  or  its  shadow,  in  four  different  po- 
sitions in  its  orbit;  when  the  moon  i,  approaches  its 
node  H,  and  the  shadow  of  the  earth  is  at  e,  it  has  no 
part  of  the  sun's  rays  intercepted;  but  if  the  earth  be 
at  F  and  the  moon  at  k,  a  small  obscuration  takes  place, 
and  the  moon  is  partially  eclipsed.  When  the  moon  is 
at  L  and  the  shadow  at  g,  it  enters  wholly  into  it, 
which  is  called  a  total  eclipse :  but  when  the  moon's 
centre  passes  through  the  centre  of  the  earth's  sha- 
dow, as  at  H,  which  can  only  happen  when  the  moon 
is  directly  in  one  of  its  nodes,  it  is  called  a  total  and 
central  eclipse. 

The  duration  of  a  central  eclipse,  or  the  time  that 
the  moon  takes,  from  entering  the  shadow  to  quitting 
it,  is  about  four  hours ;  during  two  hours  of  this  time 
the  moon  passes  through  three  times  the  length  of  its 
diameter  totally  eclipsed. 

The  moon's  diameter  is  supposed  to  be  divided  into 
twelve  equal  parts  called  digits,  and  the  magnitude  of 
a  partial  eclipse  is  denominated  by  the  number  of  parts 

2D 


^22 


Eclipses  of  the  Moon, 


that  are  obscured ;  thus,  if  the  shadow  pass  through  a 
quarter  of  the  moon's  diameter,  it  has  three  digits 
eclipsed. 

l^he  earth,  like  all  other  opake  globular  bodies^ 
which  receive  the  sun's  rays,  not  only  throws  a  dark 
converging,  or  conical  shadow  behind  it;  but  has 
likewise  a  thin  diverging  shadow  on  each  of  its  sides, 
called  the  penumbra,  which  is  occasioned  by  a  partial 
obscuration  of  light,  from  the  sun* 

For  if  s  be  the  sun,  and  e  the  earth,  receiving  its 
rays  on  its  surface ;  there  will  be  a  dark  shadow  or  total 


obscuration  in  the  cone  m,  a,  n,  which  cannot  receive 
a  ray  of  light  from  any  part  of  the  sun,  and  a  penum- 
bra or  thinner  shade  will  fall  on  each  side,  in  the  angu- 
lar parts  B  M  A,  c  N  A,  increasing  in  darkness  towards 
the  sides  ma,  na. 

For  the  penumbra  b  m  a  can  only  receive  a  partial 
light  from  the  upper  part  of  the  sun  towards  g,  which 
keeps  decreasing  till  it  terminates  at  the  side  of  the 
cone  in  the  line  g  m  a.  In  like  manner  the  penumbra 
A  N  c  is  deprived  of  any  rays  from  the  upper  part  of 
the  sun,  and  is  only  partially  illuminated  by  the  lower 
towards  h,  till  the  rays  terminate  in  the  line  h  n  a. 

The  moon  passes  through  the  penumbra  before  it 
enters  the  dark  shadow,  and  afterwards  traverses  the 
opposite  shade  before  it  resumes  its  ordinary  bright- 
ness: it  may  be  distantly  perceived  when  it  is  in  the 
outer  side  of  the  penumbra,  but  when  it  approaches 
near  to  the  dark  cone,  its  surface  is  much  more  ob- 
scured. 


Eclipses  of  the  Moon,  22^ 

Lunar  eclipses  are  visible  over  every  part  of  the 
earth,  that  has  the  moon  at  that  time  above  the  hori- 
zon ;  and  the  eclipse  appears  of  the  same  magnitude 
to  all  from  the  beginning  to  the  end.  On  the  northern 
side  of  the  equator,  the  eastern  side  of  the  moon  enters 
the  v^restern  side  of  the  shadow  and  passes  out  by  the 
eastern.  Total  central  eclipses  are  of  the  longest  du- 
ration; that  is,  when  the  diameter  of  the  earth's  sha- 
dow passes  through  the  centre  of  the  moon  in  its 
nodes;  as  the  moon  quits  its  nodes,  either  into  north 
or  south  latitude,  the  eclipses  become  more  partial  and 
of  less  duration.  The  length  of  an  eclipse,  even  in  the 
nodes,  is  not  always  the  same;  for  if  it  happen  that  the 
moon  is  in  apogee,  and  the  earth  in  aphelion,  their 
greatest  distances  from  each  other,  the  length  of  the 
eclipse  will  be  about  3  h.  57m.;  but  if  it  take  place 
when  the  moon  is  in  perigee,  and  the  earth  in  perihe- 
lion, their  nearest  distance,  then  the  duration  will  be 
8h.  37  m.  only. 

The  moon  in  the  midst  of  an  eclipse  has  usually  a 
faint  copperish  appearance ;  this  is  supposed  to  pro- 
ceed from  the  rays  of  light,  which  are  refracted  by  the 
earth's  atmosphere,  and  fall  upon  the  surface  of  the 
moon. 

The  moon's  nodes  have  a  motion  from  the  conse- 
quent to  the  antecedent  signs,  which  move  about  19^- 
degrees  in  a  year;  so  that  in  18  y.  225  d.  it  passes 
through  all  the  signs  in  the  ecliptic,  and  returns  again 
to  the  same  point.  If  the  moon's  nodes  were  fixed  in 
the  same  part  of  the  ecliptic,  there  would  be  just  half 
a  year  between  the  times  of  the  sun's  conjunction  with 
the  nodes;  therefore,  in  whatever  sign  or  month  of  the 
year  an  eclipse  should  take  place,  it  would  always 
happen  at  the  same  time  in  every  succeeding  yeai'f 
but  as  the  moon  changes  the  situation  of  its  nodes  in 
the  ecliptic  for  18  y.  225  d.  this  w^ill  be  the  period  of 
succession  before  the  same  eclipses  fal]  in  the  same 
part  of  the  ecliptic. 


224 


Eclipses  of  the  Sun. 

What  is  called  an  eclipse  of  the  sun  is  caused  by 
the  interposition  of  the  moon  between  the  sun  and  the 
earth ;  which  can  only  happen  when  the  moon  is  in  or 
near  its  conjunction.  This  seems  more  properly  call- 
ed an  eclipse  of  the  earth,  as  the  sun  loses  no  part  of 
its  brightness ;  but  ,the  intervention  of  the  moon  be- 
tween the  sun's  face  and  the  earth,  causes  a  partial 
darkness  upon  a  small  part  of  the  earth's  surface,  ac- 
companied by  a  penumbra,  or  thinner  shade,  like  that 
which  was  explained  in  the  preceding  subject. 

In  a  solar  eclipse,  let  s  represent  the  sun,  l  the 
earth,  and  m  the  moon  in  that  part  of  its  orbit  which 


is  called  in  its  conjunction,  or  between  the  earth  and 
the  sun ;  then  r ,  s,  t,  u,  is  the  moon's  conical  shadow, 
which  passes  over  z,  s,  a  small  part  of  the  earth,  and 
produces  an  eclipse,  or  withholds  the  sun's  rays  from 
that  part  of  its  surface;  and  on  each  side  of  the  conical 
shadow  is  the  penumbra,  or  shade,  which  is  caused  by 
the  partial  deprivation  of  the  rays  of  the  sun. 

Now  as  the  moon  is  so  much  less  than  the  earth,  it 
can  only  cover  a  small  part  by  its  shadow,  therefore 
those  parts  that  are  out  of  its  shade  can  perceive  no 
appearance  of  an  echpse;  even  a  central  eclipse,  that 
is,  when  the  moon's  centre  passes  through  the  diame- 
iter  of  the  sun,  can  only  be  visible  to  all  those  who 
have  the  moon  above  the  horizon. 

In  solar  eclipses,  the  moon's  shado^v  upon  the  sur- 
face of  the  earth  does  not,  in  general,  exceed  180 
miles  in  diameter;  though  the  penumbra  extends  se^ 


Eclipses  of  the  Sun.  225 

V  eral  hundred  miles  round.  If  the  ecHpse  happen  when 
the  moon  is  exactly  in  its  nodes,  it  will  cast  a  circular 
shadow  on  the  earth,  but  when  the  moon  has  northing 
or  southing  the  shade  is  elliptical. 

The  course  of  the  moon's  shadow  on  the  earth  is 
generally  from  east  to  west,  inclining  towards  the 
north,  if  it  be  in  its  ascending  node,  and  towards  the 
south  in  descending. 

The  whole  time  that  the  shadow  and  penumbra 
take  to  pass  any  given  point,  is  called  the  general 
eclipse;  the  total  eclipse  is  only  whilst  the  darkest 
part  passes  the  place. 

In  solar  eclipses,  the  face  of  the  moon  appears  co- 
vered with  a  faint  light,  which  is  attributed  to  the  re- 
flection of  the  illuminated  parts  of  the  earth.  When 
the  moon  changes  in  its  apogee,  or  greatest  distance 
from  the  earth,  its  shadow  is  not  sufficiently  long  to 
reach  to  its  surface,  and  the  sun  appears  like  a  lumi- 
nous ring  round  the  dark  body  of  the  moon,  and  forms 
what  is  called  an  annular  eclipse. 


liXPLANATION  OF  TERMS 


IN 


THE  PRECEDING  WORK, 


*'i  BERRJTIOJ\f\  in  optics,  the  deviation  or  dispersion  of  the 
rays  of  light  when  reflected  by  a  speculum  or  refracted  by  a 
lens,  by  which  they  are  prevented  from  meeting  or  uniting  in 
the  same  point,  and  then  produce  a  confusion  of  images. 

Acceleration^  the  increasing  velocity  of  heavy  bodies  as  they  fall 
towards  the  centre  of  the  earth,  by  the  force  of  gravity. 

^chromatic  telescofie^  a  species  of  refracting  telescope  which  pro- 
duces the  images  of  objects  bright,  distinct,  and  uninfected 
with  colours  about  the  edges,  through  the  whole  extent  of  a 
very  large  field  or  compass  of  view. 

Analogy^  the  comparison  of  several  ratios  of  quantities  or  num- 
bers one  to  another. 

Afihelion^  that  point  in  the  orbit  of  a  planet  in  which  it  is  at  its 
greatest  distance  from  the  sun. 

Apogee.,  that  point  of  the  orbit  of  a  planet  which  is  farthest  from 
the  earth. 

Apses^  in  astronomy,  are  the  two  points  in  the  orbits  of  the  pla- 
nets, where  they  are  at  their  greatest  and  least  distances  from 
the  sun  or  the  earth. 

Atmosphere^  a  term  used  to  signify  the  whole  of  the  fluid  mass, 
consisting  of  air,  aqueous  and  other  vapours,  electric  fluids. 
Sec,  which  surrounds  the  earth  to  a  considerable  height. 

Attenuate^  to  weaken  or  rarefy. 

Attrition^  the  action  or  rubbing  of  one  body  upon  another. 

Caloric^  supposed  to  be  that  elastic  fluid  which  produces  heat. 

Capillary  tubes^  extremely  fine  tubes,  like  hairs. 

Catoptrics^  the  science  of  reflex  vision,  or  that  part  of  optics  which 
explains  the  laws  and  properties  of  light  reflected  from  mir- 
rors or  specula. 

Centrifugal  force,  is  that  by  which  a  body  revolving  about  a  cen- 
tre, or  about  another  body,  endeavours  to  recede  from  it. 


228  EXPLANATION  OF  TERMS. 

Ccntrijietal  force,  is  that  by  which  a  moving  body  is  perpetually 
urged  towards  a  centre,  and  made  to  revolve  in  a  curve  in- 
stead of  a  right  line. 

Cohesion^  that  principle  by  which  the  p^ticlos  of  matter  in  all  bo- 
dies combine  and  stick  together. 

Collafising,  falling  together. 

Coliidun,  the  dashing  or  striking  together  of  two  bodies. 

Concave,  an  appellation  used  in  speaking  of  the  inner  surface  of 
hollow  bodies,  more  especially  of  spherical  or  circular  ones. 

Concentric,  having  the  same  centre. 

Condensation,  the  art  of  compressing  or  reducing  a  body  into  a 
less  bulk  or  spape,  by  which  means  it  is  rendered  more  dense 
and  compact. 

Congelation  or  freezing,  the  act  of  fixing  the  fluidity  of  any  liquid 
by  cold,  or  the  application  of  cold  bodies. 

Conical,  of  the  form  of  a  cone  or  sugar  loaf. 

Contact,  the  relative  state  of  two  things  that  touch  each  other,  but 
without  cutting  or  entering,  or  where  surfaces  join  each  other 
without  any  inierstice. 

Convergi.ng,  tending  to  one  point.       , 

Convex,  round  or  curved,  and  protuberant  outwards,  as  the  out- 
side of  a  globular  body. 

Corputicles,  the  minute  parts  or  particles  that  constitute  natural 
bodies. 

Curvilinear,  bounded  by  curved  lines,  as  the  circumference  of  a 
circle,  ellipsis  or  oval,  8cc. 

Densitij,  that  property  of  bodies  by  which  they  contain  a  certain 
quantity  of  matter  under  a  certain  bulk  or  magnitude. 

Diagram,  a  scheme  for  the  explanation  or  demonstration  of  any 
figure  or  its  properties. 

Diolitrics,  the  doctrine  of  refracted  vision,  or  that  -part  of  optics 
which  explains  the  effects  of  light,  as  refracted  by  passing 
through  different  mediums,  as  air,  water,  glass,  &c.  and  espe- 
cially lenses. 

Disk,  the  body  or  face  of  the  sun  or  moon,  which  appears  to  us 
as  a  circular  plane,  although  it  is  a  spherical  body. 

Diverging,  in  optics,  is  particularly  applied  to  rays  which,  issuing 
from  a  radiant  point,  or  having  in  their  passage  undergone  a 
refraction  or  reflexion,  do  continually  recede  farther  from  each 
other. 

Eccentricity  is  the  distances  between  the  centres  of  two  circles 

or  spheres  which  have  not  the  same  centre,  or  the  distance 

from  the  centre  of  an  ellipse  to  one  of  its  foci. 
Effluvium,  a  flux  or  exhalation  of  minute  particles  from  any  body, 

or  an  emanation  of  subtile  corpuscles  from  a  mixed  sensible 

body,  by  a  kind  of  motion  or  transpiration. 


EXPLANATION  OF  TERMS.  229 

Klongation^  the  removal  of  a  planet  to  the  farthest  distance  it  can 

be  from  the  sun,  as  it  appears  to  an  observer  on  the  earth. 
Equilibrium^  equality  of  weight,  equal  balance  between  two  forces 

acting  in  opposite  directions. 
Evafioration,  the  act  of  dissipating  the  humidity  of  a  body  in 

fumes  or  vapour. 
Expandon^  the  swelling  or  increase  of  the  bulk  of  a  body  when 

acted  upon  by  a  superior  degree  of  heat,  or  the  effect  produced 

by  rarefaction. 

Ferruginous^  partaking  of  the  nature  and  quality  of  iron. 
Fixity^  a  property  which  enables  a  body  to  endure  fire  and  other 

violent  agents. 
Focus^  in  optics,  is  the  point  of  convergency,  or  that  where  the 

rays  meet  after  refraction,  or  reflection. 
Friction^  the  rubbing  together  of  two  bodies. 
Fulcrum^  a  fixed  point  about  which  a  lever,  8cc.  turns  and  moves. 

Gibbous^  a  term  applied  to  the  moon  when  she  appears  more  than 
half  full  or  enlightened,  to  distinguish  her  from  the  state  when 
she  is  less  than  half  full,  or  a  crescent. 

Globules,  very  small  spherical  bodies. 

Gravity,  weight,  or  that  quality  by  which  all  heavy  bodies  tend 
towards  the  centre  of  the  earth. 

Hemisphere,  half  a  globe  or  sphere. 

Heterogeneous,  composed  of  different  kinds,  natures,  or  qualities. 

Horizontally,  parallel  to  the  horizon. 

Horizon,  a  circle  dividing  the  visible  part  of  the  earth  and  hea- 
vens from  that  which  is  invisible. 

Hypothesis,  m  philosophy,  denotes  a  kind  of  system  laid  down 
from  our  own  imagination,  by  which  to  account  for  some  phe- 
nomena or  appearances  of  nature. 

Ignite,  to  kindle  or  generate  fire. 

Impulsion,  XhvvisiiYi^  forwards  or  driving  on. 

injiection,  in  optics,  called  also  diffraction  and  deflection  of  the 
rays  of  light,  is  a  property  of  them,  by  reason  of  which,  when 
they  come  within  a  certain  distance  of  any  body,  they  will  be 
either  bent  from  or  towards  it,  being  a  kind  of  imperfect  re- 
flection or  refraction. 

Intensity,  the  degree  or  rate  of  the  power  or  energy  of  any  quality. 

Interstices,  spaces  between,  or  where  parts  are  not  in  contact. 

Lens,  a  piece  of  glass  or  other  transparent  substance,  so  formed 
that  the  rays  of  light  in  passing  through  it  have  their  direction 
changed. 

2E 


230  EXPLANATION  OF  TERMS. 

Maximum,  the  greatest  quantity,  force,  8cc.  which  can  take  place 

under  certain  circumstances. 
Medium,  denotes  that  space,  or  region,  of  fluid,  &c.,  through 

which  a  body  passes  in  its  motion  towards  any  point. 
MeHdian,  in  astronomy,  is  a  great  circle  of  tlie  celestial  sphere 

passing  through  the  poles  of  the  world,  and  both  the  zenith  and 

nadir,  crossing  the  equinoctial  at  right  angles,  and  dividing  the 

sphere  into  two  equal  parts  or  hemispheres,  the  one  eastern 

and  the  other  western. 
Meridian,  in  geography,  is  a  great  circle  passing  through  the 

poles  of  the  earth,  and  any  given  place  the  meridian  of  which 

it  is. 
Momentum,  the  quantity  of  motion  in  a  moving  body. 

Oblate,  flatted  at  the  poles. 
Ofiake,  dark,  thick,  not  transparent. 
Orbit,  the  course  in  which  any  planet  moves. 
Oscillation,  or  vibration,  is  the  reciprocal  ascent  and  descent  of  a 
pendulum. 

Pendulum,  any  heavy  body  so  suspended  as  that  it  may  swing 
backwards  and  forwards  about  some  fixed  point  by  the  force  of 
gravity. 

Penumbra,  a  faint  or  partial  shade  in  an  eclipse,  observed  be- 
tween the  perfect  shadow  and  the  full  light. 

Perilielion,  that  point  in  the  orbit  of  a  pUmet  or  comet  which  is 
nearest  to  the  sun.  In  which  sense  it  stands  opposed  to  Aphe- 
lion, which  is  the  highest  or  most  distant  point  from  the  sun. 

Perigee,  is  that  point  in  the  heavens  in  which  the  sun  or  any 
planet  is  nearest  to  the  earth. 

Phases,  the  various  appearances  or  quantities  of  illumination  of 
the  Moon,  Venus,  Mercury,  and  the  other  planets,  by  the  sun. 

Phenomenon,  a  singular  appearance  in  nature. 

Porosity,  the  quality  of  being  porous  or  full  of  small  holes. 

Quadrature,  in  astronomy,  that  aspect  or  position  of  the  moon 
when  she  is  90°  distant  from  the  sun. 

Radiant,  any  point  from  which  rays  proceed. 

Ramous,  full  of  branches  or  fibres. 

Ratio,  the  rate  or  proportion  which  several  quantities  or  numbers 
bear  to  each  other. 

Refrangible,  capable  of  being  refracted  or  bent. 

Reflection  of  the  rays  of  light,  is  their  motion  after  being  repel- 
led or  reflected  from  the  surface  of  bodies. 

R<fraction  of  light,  is  an  inflection  or  deviation  of  the  rays  from 


EXPLANATION  OF  TERMS.  23 1 

their  rectilinear  course  on  passing  obliquely  out  of  one  me- 
dium into  another  of  a  different  density. 

Reflexible,  capable  of  being  reflected. 

Rotatory^  turning  round. 

Retardation^  the  act  of  retarding,  that  is,  of  delaying  the  motion 
or  progress  of  a  body,  or  of  diminishing  its  velocity. 

Retina,  the  expansion  of  the  optic  nerve. 

Satellites,  certain  secondary  planets  moving  round  the  other  pla- 
nets as  the  moon  moves  round  the  earth. 

Segment,  is  a  part  cut  off  the  top  of  a  figure  by  a  line  or  plane ; 
the  segment  of  a  circle  is  a  figure  contained  between  a  chord 
and  an  arc  of  the  same  circle. 

Semidiameter,  a  line  drawn  from  the  centre  of  a  circle  to  any  part 
of  its  circumference. 

Sine,  a  right  line  drawn  from  one  extremity  of  an  arc,  perpendi- 
cular to  the  radius,  drawn  to  the  other  extremity  of  it. 

Solstice,  is  the  time  when  the  sun  is  at  the  greatest  distance  from 
the  equator.  There  are  two  solstices  in  each  year,  called  the 
summer  solstice  and  winter  solstice. 

Sfieculum,  any  polished  body  which  reflects  the  rays  of  light. 

Tangent,  a  right  line  drawn  on  the  outside  of  a  circle,  and  just 
touching  its  circumference. 

Vacuum,  a  term  used  in  philosophy  to  denote  a  space  entirely 
void  of  all  matter. 

Vertex,  the  top  of  any  line  or  figure ;  in  astronomy,  that  point  in 
the  heavens  immediately  over  our  heads. 

Vesicle,  a  small  cuticle  filled  or  inflated,  or  a  very  small  bladder. 

Visual  rays  are  lines  of  light  conceived  to  come  from  an  object  to 
the  eye. 

Volatilize,  to  separate  the  particles  of  a  body,  or  to  make  it  eva- 
porate. 


THE  END, 


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